depti 


LIBRARY 

OF  THE 

UNIVERSITY  OF  CALIFORNIA. 

Class 


UNIVERSITY  OF   CALIFORNIA 

LIBRARY 

OF  THE 

DEPARTMENT   OF   PHYSICS 


Received 

/  /f  AX  / 

Accessions  No. K..J.  * Book  No..-.L. 


SCIENTIFIC  MEMOIRS 

EDITED    BY 

J.  S.  AMES,  PH.D. 

PROFESSOR   OF   PHYSICS    IN   JOHNS   HOPKINS   UNIVERSITY 


XIII. 
THE  FOUNDATIONS  OF  STEREO-CHEMISTRY 


THE  FOUNDATIONS 

OF 

STEREO  CHEMISTRY 

MEMOIRS   BY  PASTEUR,   VAN'T   HOFF, 
LEBEL,  AND   WISLICENUS. 


TRANSLATED    AND    EDITED     BY 

GEORGE  M.  RICHARDSON,  PH.D., 

PROFESSOR    OF    ORGANIC    CHEMISTRY 
IN  LELAND    STANFORD   JUNIOR    UNIVERSITY. 


NEW  YORK  •:•  CINCINNATI  -:-  CHICAGO 

AMERICAN  BOOK   COMPANY 


COPYRIGHT,  1901,  BY 
AMERICAN  BOOK  COMPANY. 


Entered  at  Stationers'1  Hall,  London. 

Stereo-chemistry. 

W.  P.   i 


^yxk.  ~, 


PREFACE. 

STEREO-CHEMISTRY,  or  as  it  has  sometimes  been  called, 
Chemistry  in  space,  or  Geometrical  chemistry,  is  that  branch  of 
chemistry  in  which  the  relative  position  of  the  atoms  in  space 
is  taken  into  consideration. 

The  stereo-chemical  theories,  like  most  useful  scientific 
theories,  are  the  result  of  the  necessary  expansion  of  the  older 
views  to  explain  newly  discovered  facts. 

The  plane  structural  formulas  based  upon  conceptions  of 
simple  atom  linking  without  regard  to  the  actual  arrangement 
of  the  atoms  with  respect  to  one  another,  were  for  a  time  suffi- 
cient to  meet  the  requirements  of  the  known  facts.  While 
chemists  recognized  that  the  atoms  of  the  molecule  must  have 
some  definite  arrangement  with  respect  to  one  another,  it  was 
believed  by  most  chemists  that  an  actual  knowledge  of  this 
arrangement  lay  wholly  outside  of  the  field  of  experimental 
study. 

The  first  suggestion  leading  to  the  views  now  held  came  from 
Pasteur,  (See  Pasteur's  article  here  given)  in  1860  when  he 
stated  that  the  difference  between  the  active  tartaric  acids 
must  be  due  to  the  asymmetric  nonsuperposable  character  of 
their  molecules.  In  1873  Wislicenus,  after  a  thorough  study  of 
the  active  lactic  acid  derived  from  meat  and  the  inactive  lactic 
acid  from  milk,  announced  that  the  difference  between  these 
two  acids  must  be  accounted  for  by  a  difference  in  the  arrange- 
ment of  their  atoms  in  space. 

The  first  definite  suggestion  as  to  what  this  arrangement 
might  be  was  made  by  van't  Hoff  in  a  Dutch  pamphlet  (here 
translated)  published  in  September,  1874.  Almost  at  the 
same  time,  in  November,  1874,  LeBel  published  a  paper  (here 
translated)  in  which  he  developed,  independently,  essentially 
the  same  views. 

A  little  later,  in  May,  1875,  after  some  further  elaboration  of 
his  ideas,  van't  Hoff  published  them  in  French  under  the  title 

V 

222562 


PREFACE. 

"La  Chimie  dans  1'Espace."  In  1877  this  pamphlet  with 
further  additions  was  translated  into  German  by  F.  Herrmann. 
The  theory  at  first  did  not  meet  with  a  cordial  reception  at  the 
hands  of  chemists,  but  on  the  contrary  received  scant  recogni- 
tion.1 

As  time  went  on,  however,  the  necessity  for  some  expansion  of 
the  older  views  became  greater  and  greater  and  chemists  began 
to  turn  to  the  theory  of  van't  Hoff  and  LeBel  as  a  possible 
means  of  relief.  The  final  impetus  in  this  direction  was  given 
in  1887  by  Wislicenus  when  he  published  his  paper  (here  trans- 
lated) in  which  he  took  up  this  theory  and  applied  it  system- 
atically and  with  the  greatest  success  to  the  explanation  of  a 
series  of  extremely  puzzling  relationships. 

In  consequence  of  this  paper  attention  was  directed  anew  to 
the  theory  and  a  considerable  number  of  chemists  turned  their 
attention  to  its  experimental  confirmation.  Asa  result  of  this 
work  the  stereo-chemical  theories  are  at  present  quite  generally 
accepted  and  they  stand  among  the  most  fruitful  working  hy- 
potheses yet  introduced  into  the  chemistry  of  the  carbon  com- 
pounds. 

Considerable  progress  has  already  been  made  toward  the 
understanding  of  the  stereo-chemistry  of  nitrogen  compounds, 
and  a  beginning  has  been  made  in  the  stereo-chemistry  of  a  few 
inorganic  compounds. 

G.  M.  R. 

1  See  Kolbe's  criticism  of  the  theory.  Jour,  fiir  Prakt.  Chem.  1877, 
[2]  15,  473. 


vi 


GENERAL,  CONTENTS. 

PAGE. 

Preface .        .     .  .  v 

The    Asymmetry    of  Naturally  Occurring  Organic  Com- 
pounds.   By  Louis  Pasteur 1 

Biographical  Sketch  of  Pasteur   . 33 

Structural  Formulas  in  Space.    By  J.  H.  van't  Hoff      .        .  35 

Biographical  Sketch  of  van't  Hoff 46 

Relation  between  Atomic  Formulas  of  Organic  Compounds 
and  the  Rotatory  Power  of  their  Solutions.     By  J.  A. 

LeBel 47 

Biographical  Sketch  of  LeBel 60 

Space  Arrangement  of  the  Atoms  in  Organic  Molecules  and 
the  Resulting  Geometrical  Isomerism  in  Unsaturated 

Compounds.    By  Johannes  Wislicenus    ....  61 

Biographical  Sketch  of  Wislicenus 132 

Bibliography •  133 


ON    THE    ASYMMETRY    OF    NATURALLY 
OCCURRING  ORGANIC  COMPOUNDS. 

BY 

LOUIS  PASTEUR. 

Tivo  Lectures  delivered  before  the  Chemical  Society  of  Paris  on 
the  20th  of  January  and  the  3d  of  February,  1860. 


CONTENTS. 

FIRST  LECTURE.  PAGE. 

Polarized  Light .........  3 

Hemihedrism 6 

Crystal  form  of  Tartaric  Acid  and  its  Salts  ...  8 
Mitscherlich's  Statement  Concerning  the  Sodium-Ammonium 

Salts  of  Tartaric  and  Racemic  Acids  .  .  .11 

Historical  Reminiscences 13 

Racemic  Acid  a  Compound  of  Dextro-  and  Lcevo- Tartaric 

acids         ........-,  14 

Relationship  between  Dextro- Tartar ic  and  Lcevo- Tartaric 

Acids 17 

SECOND  LECTURE. 

All  Molecules  are  either  Symmetrical  or  Asymmetric  .  ,18 
Ttie  Molecule  of  Tartaric  Acid  is  Asymmetric  ,  .  .19 
Mineral  Bodies  have  Symmetrical  Molecules,  Organic  Bodies 

Have,  for  the  Most  Part,  Asymmetric  Molecules  .  20 
Distinction  between  Crystal  Asymmetry  and  Molecular 

Asymmetry 20 

Asymmetry  a  Direct  Organic  Principle  not  Possessed  by 

Artificial  Bodies       .......     21 

Four  Well  Marked  Arrangements  for  the  Atom  Groups  ,  26 
Transformation  of  One  Active  Modification  into  its  Optical 

Opposite  by  the  Action  of  Heat 27 

Optical  Isomers  behave  Differently  toward  Other  Active 

Bodies 28 

Splitting  of  Racemic  Acid  by  Active  Bases  ,  .  .  .29 
Asymmetric  Force  Active  at  the  Origin  of  Natural  Organic 

Compounds       .         .         .         .         .         .         .         .30 

Physiological  Action  of  Asymmetric  Compounds  .  .  .31 
Biographical  Sketch  of  Pasteur 33 


THE    ASYMMETRY   OF   NATURALLY    OC- 
CURRING ORGANIC  COMPOUNDS. 

BY  LOUIS  PASTEUB. 

Two  lectures  delivered  before  the  Chemical  Society  of  Paris, 
oh  January  20,  and  February  3,  1860. 

AT  the  end  of  the  year  1808  Mains  discovered  that  light 
which  was  reflected  from  opaque  or  transparent  bodies  pos- 
sessed new  and  surprising  properties,  which  distinguished  it 
from  the  light  that  proceeded  directly  from  illuminating 
bodies.  Malus  called  the  change  which  the  light  suffered  by 
its  reflection,  polarization.  Later  the  reflection  plane  itself, 
i.  e.,  the  plane  passing  through  the  incident  ray  and  the  normal 
to  the  reflecting  surface,  was  designated  the  plane  of  polariza- 
tion of  the  rays.  Malus  did  not,  however,  limit  here  his  dis- 
coveries with  regard  to  polarized  light.  It  had  been  known  for 
a  considerable  time  that  a  direct  ray  of  light  in  passing  through 
acalcite  rhombohedron  was  divided  into  two  rays  of  equal  in- 
tensity. A  flame  when  observed  through  such  a  rhombohedron 
always  appeared  double  and  both  images  were  of  equal  clearness. 

Huygens  and  Newton  had  earlier  found  that  light  which  had 
passed  through  Iceland  spar  differed  from  direct  light. 

When  one  or  the  other  of  the  two  images  above  mentioned  is 
examined  through  a  new  rhombohedron,  doubling  of  the  image 
does  not  always  take  place  ;  and  when  doubling  of  the  image 
does  occur  the  two  new  images  no  longer  possess  equal  intensity. 

Light  which  has  passed  through  a  doubly  refracting  crystal 
is  thus  different  from  natural  or  direct  light. 

It  was  proved  by  Malus  that  the  change  which  occurs  in 
light  by  its  double  refraction  is  identical  with  that  which  is 
produced  by  its  reflection  from  the  surface  of  transparent  or 
opaque  bodies  ;  in  other  words,  that  both  the  ordinary  and  ex- 
traordinary rays  which  emerge  from  a  doubly  refracting  crystal 

3 


MEMOIRS    ON 

are  polarized.  This  fruitful  discovery  was  made  by  Malus,  and 
from  the  first  was  presented  so  clearly,  and  advanced  with  such 
moderation  and  precision  in  experiment  and  language,  that 
when  the  paper  is  read  it  appears  as  though  it  had  been  written 
at  the  present  time.  Yet  he  was  not  permitted  to  continue 
his  work  ;  in  the  year  1812,  he  died  at  the  early  age  of  thirty- 
seven  years. 

Fortunately  for  the  science  two  celebrated  physicists,  Biot 
and  Arago,  both  at  that  time  young  and  in  their  prime,  took 
up  this  heritage;  and  by  means  of  brilliant  discoveries  they 
soon  made  great  advance  in  this  new  department  of  science 
which  had  been  established  by  Mains. 

In  the  year  1811,  Arago  found  that  when  a  ray  of  polarized 
light  passes  perpendicularly  through  a  quartz  plate  which  has 
been  cut  in  a  direction  at  right  angles  to  its  axis,  and  is  then 
analyzed  by  means  of  a  calcite  rhombohedron,  two  images  are 
produced  in  all  positions  of  the  rhombohedron,  and  the  two  im- 
ages appear  in  complementary  colors.  If  the  thickness  of  the 
calcite  is  not  sufficient  to  produce  complete  separation  of  the  two 
rays,  then  the  portion  of  the  image  covered  by  both  rays  is  white. 
This  experiment  showed  a  twofold  anomaly  when  compared 
with  the  ordinary  laws  of  doubly  refracting  crystals.  Every  other 
uniaxial  crystal,  when  cut  perpendicular  to  the  axis,  produces 
two  white  images  instead  of  colored  images;  and  in  two  posi- 
tions of  the  rhombohedron  at  right  angles  from  one  another, 
only  one  image  is  produced.  Arago's  conclusions  were  that  the 
results  of  the  foregoing  experiments  were  exactly  the  same  as 
would  occur  if  the  different  colored  components  of  the  incident 
white  ray  were  polarized  in  different  planes  as  they  emerge 
from  the  quartz  plate.  Arago  did  not  return  to  the  investiga- 
tion of  these  phenomena.  Their  physical  laws  were  determined 
wholly  by  Biot  since  1813  ;  while  this  work  is  entirely  separated 
from  that  of  Arago,  there  is  danger  of  its  being  confounded 
with  it. 

Biot  polarized,  one  after  the  other,  the  simple  rays  of  the 
spectrum  and  found  that  the  original  plane  of  polarization  was 
rotated  through  an  angle  which  was  proportional  to  the  thick- 
ness of  the  plate;  that  this  angle  changed  with  each  of  the 
simple  colors,  and  increased,  according  to  a  definite  law,  with 

4 


S  T  E  Ji  E  0  -  C  H  E  M  I S  T  B  Y . 

the  refrangibility  of  the  raj.  Biot  made  the  farther  notewor- 
thy observation,  that,  with  plates  taken  from  different  quartz 
prisms,  some  turned  the  plane  of  polarization  toward  the  right, 
while  others  turned  it  toward  the  left;  but  otherwise  according 
to  exactly  the  same  laws. 

But  Biotas  greatest  discovery  in  this  field  is,  without  doubt^ 
his  observation  that  certain  natural  organic  substances  rotate 
the  plane  of  polarization;  as,  oil  of  turpentine,  solutions  of 
sugar,  camphor  and  tartaric  acid.  The  first  communication  of 
these  facts  is  found  in  the  proceedings  of  the  "  Societe  philo- 
matique  "  for  December,  1815. 

In  order  properly  to  understand  this  lecture  we  must  call  at- 
tention to  the  power  of  rotation  which  exists  in  tartaric  acid 
and  the  lack  of  this  property  in  paratartaric  or  racemic  acid, 
one  of  the  isomers  of  tartaric  acid. 

There  are  thus  organic  substances,  fluid  or  soluble  in  water, 
which  possess  the  power  of  rotating  the  plane  of  polarization 
and  in  this  respect  resemble  quartz.  It  is,  however,  important 
to  note  that  this  analogy  with  quartz  is  only  apparent.  In  both 
cases  we  have  to  do  with  the  rotation  of  the  plane  of  polariza- 
tion, but  the  phenomena  are,  nevertheless,  entirely  different. 
In  order  that  quartz  shall  be  active  it  must  be  crystallized.  In 
the  dissolved  form  or  in  the  solid  but  nncrystallized  condition, 
it  is  without  action.  It  must  not  only  be  crystallized,  but  the 
plates  must  be  cut  perpendicular  to  its  axis  ;  so  soon  as  the 
plates  are  turned  in  the  direction  of  the  ray,  the  rotation  be- 
comes weaker  and  at  last  entirely  disappears.  Sugar  is  active 
(and  what  I  say  of  sugar  applies  also  to  the  other  organic  prod- 
ucts) but  the  sugar  must  be  dissolved,  or  solid  arid  amorphous; 
as,  sugar-candy.  In  the  crystalline  condition  it  is  impossible 
to  observe  any  action.  The  tube  with  the  sugar  solution  may 
be  inclined  but  so  long  as  the  layer  remains  of  the  same  thick- 
ness there  is  no  change  in  the  amount  of  the  rotation.  Even 
though  the  solution  be  actively  stirred  by  clockwork,  the  phe- 
nomena remain  the  same. 

Hence,  from  the  beginning,  Biot  drew  the  conclusion  with 
perfect  certainty,  that  the  action  of  organic  bodies  was  a 
characteristic  of  the  molecules,  and  was  due  to  the  individual 
constitution  of  these  smallest  particles.  But  with  quartz,  the 

5 


MEMOIRS     ON 

phenomena  are  due  to  the  manner  of  arrangement  of  the  crys- 
talline particles. 

These  are,  if  I  may  be  allowed  to  use  the  expression,  the 
physical  preliminaries  of  the  investigations  concerning  which  I 
wish  to  speak.  We  will  now  proceed  with  the  mineralogical 
preliminaries. 

II. 

HEMIHEDKISM  is  a  crystallographic  property  which  in  its 
outward  characteristics  is  readily  recognized.  If  we  take,  for 
example,  a  mineral  species  which  crystallizes  regularly,  it  may, 
as  is  well  known,  assume  different  forms  which  are  determined 
by  the  law  of  symmetry;  a  law  which  is  so  universal  that  it  may 
almost  be  assumed  as  a  physical  axiom.  By  means  of  this  law 
from  any  one  form  all  related  forms  may  be  derived  by  chang- 
ing or,  as  Rome  de  Lisle  expressed  it,  by  truncating  all  identi- 
cal parts  at  the  same  time  and  in  the  same  manner.  Those 
edges  are  called  identical  which  are  formed  by  the  meeting  of 
identical  faces  at  equal  interfacial  angles,  and  identical  solid  or 
crystal  angles  are  those  which  are  formed  by  the  meeting  of  an 
equal  number  of  identical  edges.  In  the  cube,  for  example, 
there  is  but  one  kind  of  crystal  angle  and  but  one  kind  of  edge. 
If  one  of  the  crystal  angles  is  truncated  by  a  plane  equally  in- 
clined to  the  three  faces  of  the  cube,  the  other  seven  crystal 
angles  will  be  modified  in  the  same  way,  as  is  seen  in  crystals 
of  alum  and  blende,  and  in  general  with  all  crystalline  bodies 
of  the  regular  system. 

Let  us  now  consider  a  plane  rhombic  prism,  the  eight  edges 
of  the  end  planes  are  identical.  If  one  of  these  is  truncated, 
then  the  other  seven  will  be  truncated.  The  four  vertical  edges 
are  of  another  kind  and  will  in  general  not  be  modified  at  the 
same  time  with  the  other  eight  edges,  or,  if  they  are  modified, 
the  change  may  be  of  a  different  kind. 

These  examples  are  enough  to  illustrate  the  laws  of  symmetry 
and  its  applications.  Now  it  is  easy  to  give  a  clear  idea  of  hem- 
ihedrism.  Examples  have  long  been  known,  even  Hauy  knew 
several  celebrated  cases  of  this  kind,  in  which  only  half  of  the 
identical  parts  are  changed  in  the  same  way  at  the  same  time. 
This  is  called  hemihedrism.  The  cube  which  was  before 

6 


STEKEO-CHEMISTKY. 

changed  in  all  eight  of  its  crystal  angles  is  now  changed  in  but 
four.  A  known  example  of  this  kind  is  Boracite.  These 
changes  are  such  that  if  one  imagines  the  truncating  planes  to 
be  extended  until  they  intersect  one  another  a  regular  tetrahe- 
dron is  produced.  Had  the  change  been  upon  the  other  four 
corners  of  the  cube,  another  regular  tetrahedron  would  have 
been  produced  identical  in  form  with  the  first,  and  differing 
from  it  only  in  its  position  on  the  cube. 

Similarly  we  find  it  with  our  rhombic  prism.  With  some 
species  only  half  of  the  edges  are  truncated  and  here,  too,  if 
the  truncating  planes  which  are  at  opposite  edges  of  each  end 
plane  and  at  dissimilar  edges  of  the  two  end  planes,  are  suffi- 
ciently extended,  they  produce  a  tetrahedron.  Here,  as  with 
the  cube,  two  tetrahedrons  are  possible,  which  differ  in  their 
position  with  regard  to  the  prism,  according  as  one  assumes  the 
extension  of  one  or  the  other  group  of  truncating  planes.  But 
here  the  two  tetrahedrons  are  not  absolutely  identical.  They 
are  asymmetric  tetrahedrons  whicli  cannot  be  brought  into 
identical  positions,  which  cannot  be  superposed.  These  illus- 
trations will  be  sufficient  in  order  to  understand  hemihedrism 
and  the  term  hemihedral  forms  or  hemihedral  planes. 

Quartz  is  one  of  the  few  mineral  species  in  which  hemihe- 
drism was  early  observed  by  Hauy.  The  common  form  of  this 
mineral  is  well  known,  it  is  a  large  hexagonal  prism  with  hex- 
agonal pyramids.  It  is  clear  that  the  crystal  angles  formed  by 
the  prism  and  pyramids  are  identical  with  one  another  and,  that 
when  one  is  truncated,  the  others  should  also  be  truncated. 
This  is,  in  fact,  the  case  and  the  planes  are  in  Mineralogy  called 
rhombohedral  faces. 

Hauy  first  observed  that  in  certain  crystals  there  appeared  a 
plane  very  different  irom  these,  which  he  designated  x;  it  had 
an  inclined  position,  but  still,  did  not  as  indicated  by  the  law 
of  symmetry,  occur  twice.  There  is  another  characteristic  of 
this  crystal  which  has  not  escaped  crystallographers  ;  namely, 
that  this  face  is  sometimes  inclined  in  one  direction  and  some- 
times in  the  other.  Hauy,  who  loved  to  give  to  every  variety 
of  a  species  a  name,  has  called  the  quartz  crystals  which  have 
the  face  x,  plagihedral  ;  and,  indeed,  called  those  crystals 
which  had  the  face  x  inclined  toward  the  right  dextro-plagi- 

7 


MEMOIRS     ON 

hedral  ;  while  laevo-plagihedral  designated  those  crystals  in 
which  this  face  was  inclined  in  the  opposite  direction.  The 
occurrence  of  this  face  appears  almost  accidental  ;  now  it  is 
present  and  now  it  is  not.  In  the  same  crystal  some  corners 
may  have  the  face  x,  while  other  corners  where  it  would  he  ex- 
pected do  not  have  it.  Frequently  dextro-  and  laevo-plagihedral 
forms  are  found  on  the  same  crystal.  In  spite  of  this  fact  crys- 
tallographers  are  agreed  that  quartz  is  hemihedral,  but  that  in 
such  cases  the  opposite  forms  of  hemihedrism  occur  at  the 
same  time. 

Here  I  must  introduce  a  very  fruitful  idea  which  was  first 
communicated  to  the  Royal  Society  of  London  in  the  year  1820, 
by  Sir  John  Herschell.  As  I  have  already  mentioned,  Biot 
had  made  the  noteworthy  observation  that  some  quartz  crystals 
turn  the  plane  of  polarized  light  toward  the  right,  while  others 
turn  it  toward  the  left.  Herschell  made  a  direct  connection 
between  the  observation  of  Biot  and  Hauy's  crystallographic 
results ;  and  the  thought  has  been  fully  confirmed  by  exper- 
iment, in  that  those  crystals  which  had  the  face  x  in  the  same 
position,  as  for  example  the  dextroplagihedral  forms,  would  all 
turn  the  plane  of  polarization  of  light  in  the  same  direction. 

I  have  now  given  you  the  chief  facts  which  lead  up  to  the 
investigations  that  I  wish  to  communicate  to  you. 

III. 

WHEN"  I  began  to  apply  myself  to  my  own  work  I  undertook 
a  thorough  study  of  crystals,  assuming  that  it  would  bet  useful 
to  me  in  my  own  investigations.  It  appeared  to  me  that  the 
simplest  method  of  accomplishing  this  was  to  start  my  studies 
by  a  thorough  study  of  crystal  forms,  to  repeat  all  measure- 
ments and  to  compare  my  results  with  the  original  ones.  In 
the  year  1841  de  La  Provostaye,  whose  thoroughness  is  well 
known,  published  a  brilliant  work  upon  the  crystal  form  of 
tartaric  acid,  racemic  acid,  and  their  salts.  I  repeated  this 
work.  I  allowed  tartaric  acid  and  its  salts  to  crystallize  and 
then  determined  their  crystal  form.  But  during  the  work  I 
observed  that  the  learned  physicist  had  overlooked  one  very 
important  fact.  In  all  the  tartaric  acid  salts  which  I  studied 
I  noted  undoubted  evidences  of  hemihedral  faces.  This  char- 

8 


S  T  E  K  E  0-C  HEMISTRY. 

acteristic  of  the  tartrates  was  not  a  very  prominent  one.  It 
can  therefore  readily  be  understood  that  it  had  never  before 
been  observed. 

If  with  any  given  crystal  it  was  doubtful,  it  could  always  be 
rendered  apparent  by  recrystallizing  it  under  slightly  different 
conditions.  In  many  cases,  indeed,  the  crystal  had  all  the  faces 
indicated  by  the  law  of  symmetry,  but  hemihedrism  was  recog- 
nized by  the  unequal  development  of  one  half  of  the  faces. 
This  is  the  case  for  example  with  the  ordinary  tartar  emetic. 
The  difficulty  of  recognizing  hemihedrism  is  much  increased  by 
the  frequent  irregularities  of  crystals  which  have  not  been 
formed  entirely  free.  Such  cases  result  in  distortion,  interrup- 
tion of  crystallization  in  one  direction  or  another,  accidental 
arrest  of  faces,  etc. 

The  determination  of  hemihedrism,  especially  with  crystals 
which  have  been  formed  in  the  laboratory,  requires  usually 
most  attentive  study.  In  addition  there  is  the  case  of  hemihe- 
drism which  manifests  itself  through  the  inner  structure  of 
the  body  and  is  not  necessarily  outwardly  visible,  all  the  forms 
of  regular  crystals  are  met  with  in  this  group. 

However  it  may  be,  I  repeat,  I  found  all  tartrates  to  be 
hemihedral.  This  observation  without  what  follows  would 
have  remained  fruitless. 

Let  a.  5,  c  be  the  axes  of  a  crystal  of  a  tartaric  acid  salt  ; 
a,  j3,  y  tTie  angles  between  the  crystal  axes. 

These  angles  are  usually  right  angles  or  deviate  but  little 
from  right  angles.  Moreover  the  ratio  between  two  of  the 
axes,  for  example  between  a  and  #,  is  nearly  the  same  for  all 
tartrates  ;  c  only  varies  to  any  extent. 

Apparently  it  is  a  weak  kind  of  isomorphism  found  in  all 
tartrates.  It  might  be  said  that  the  tartaric  acid  group  pre- 
dominates and  forces  a  common  character  upon  the  different 
forms  in  spite  of  the  difference  in  the  other  element  in  the 
compound.  It  follows,  therefore,  that  the  salts  of  tartaric 
acid  have  a  common  form,  and  that  it  is  possible  to  give  them 
similar  orientation  by  placing  the  axes  a  and  b  in  like  po- 
sitions. If  the  crystals  are  compared  after  being  thus  oriented 
it  is  found  that  the  arrangement  of  the  hemihedral  faces  is 
always  the  same.  These  facts,  which  are  the  starting  point  of 
B  9 


MEMOIRS     ON. 

all  my  later  experiments,  may  be  summed  up  in  the  words  : 
The  tartrates  are  hemihedral,  and  always  in  the  same  sense. 

Guided,  on  the  one  hand  by  the  observations  of  Biot,  that 
tartaric  acid  and  all  of  its  compounds  possessed  molecular 
rotation,  and  on  the  other  by  the  relationship  suggested  by 
Herschell,  and  thirdly,  by  the  learned  views  of  Delafosse,  who 
conceived  that  hemihedrisrn  was  a  result  of  crystallographic 
laws  and  not  a  chance  phenomenon,  I  concluded  that  there  was 
a  relation  between  the  hemihedrism  of  the  tartrates  and  their 
optical  activity.  It  is  important  here  to  follow  closely  the  line 
of  thought.  Hauy  and  Weiss  recognized  the  occurrence  of 
hemihedral  faces  upon  quartz,  and  that  these  faces  in  certain 
individuals  lay  to  the  right  and  in  others  to  the  left.  Biot 
had  found  that  quartz  crystals,  as  regards  their  optical  activity, 
could  be  divided  into  two  groups;  one  turned  the  plane  of 
polarization  toward  the  right,  the  other  followed  the  same  law, 
except  that  it  turned  the  plane  of  polarization  toward  the 
left. 

Then  comes  Herschell,  with  the  thought  that  unites  these 
two  isolated  facts,  it  is:  Plagihedral  forms  of  one  kind  all 
rotate  the  plane  of  polarization  in  the  same  direction  and  of 
the  other  kind  rotate  it  in  the  opposite  direction. 

I,  for  my  part,  found  that  all  tartrates  are  plagihedral,  if  I 
may  so  express  it,  and  that  they  are  all  of  the  same  kind.  I 
dared  to  assume,  therefore,  that  here,  as  with  quartz,  there 
was  a  connection  between  the  hemihedrism  and  circular  polar- 
ization. In  spite  of  this  the  striking  difference  between  the 
circular  polarization  of  quartz  and  tartaric  acid,  to  which  I 
have  already  called  attention,  could  not  be  ignored.  It  was 
necessary  for  us,  owing  to  new  facts  and  their  relation  to  one 
another  which  I  am  about  to  point  out,  to  form  an  hypothesis 
(for  it  is  not  more  as  yet)  concerning  the  connection  between 
hemihedrism  and  circular  polarization  in  the  tartrates. 

As  I  very  much  wished  to  find  some  experimental  support 
for  these  speculations,  my  first  thought  was  to  examine  the 
numerous  crystallizable  organic  substances  which  possessed 
optical  activity,  for  hemihedral  crystal  forms,  which,  in  spite 
of  Herschell's  suggestions,  had  not  yet  been  done. 

These  studies  had  the  wished  for  result.     I  investigated  the 

10 


S  T  E  BE  0-0  H  E  M  I  S  T  ft  Y. 

crystal  form  of  racemic  acid,  which  had  been  shown  by  Biot  to 
be  entirely  inactive  toward    polarized  light. 

None  of  these  crystals  showed  hemihedrism.  In  this  way 
the  idea  of  a  relation  between  hemihedrism  and  the  molecular 
rotation  of  organic  products  won  new  support.  Soon  I  was 
able  through  an  unexpected  discovery  to  make  this  clear. 

IV. 

IT  is  necessary  that  I  make  you  acquainted  with  an  important 
communication  of  Mitscherlich's  which  was  laid  before  the 
French  Academy  of  Science  by  Biot.  I  quote  his  words  : 
"The  racemic  acid  and  the  tartaric  acid  sodium-ammonium- 
double  salts  have  the  same  chemical  composition,  the  same 
crystal  form  with  equal  angles,  the  same  specific  gravity,  the 
same  double  refraction  and  in  consequence  of  this  their  optical 
axes  form  the  same  angle.  Their  water  solutions  have  the 
same  refraction.  But  the  dissolved  tartaric  acid  salt  rotates 
the  plane  of  polarization,  and  the  racemic  acid  salt  is  indiffer- 
ent, as  has  been  found  by  Biot  for  the  whole  series  of  salts." 
"But,"  continues  Mitscherlich,  "the  nature  and  the  number 
of  atoms,  their  arrangement,  and  their  distance  from  one  another 
are  the  same  in  both  bodies."  This  conclusion  at  the  time  of 
its  publication  especially  concerned  me.  I  was  at  that  time  a 
scholar  at  the  Ecole  Normale,  and  was  studying  during  my 
spare  time  the  interesting  investigations  upon  the  molecular 
constitution  of  bodies;  and  was,  as  I  at  least  believed,  begin- 
ning to  understand  the  generally  accepted  principles  of  phys- 
icists and  chemists.  The  above  mentioned  communication 
upset  all  of  my  ideas.  What  similarity  in  all  respects!  Are 
there  two  bodies  the  properties  of  which  have  been  more  thor- 
oughly and  carefully  compared  ?  Can  there  be  in  the  present 
state  of  the  science  two  bodies  so  entirely  similar  without 
being  identical  ?  Mitscherlich  himself  tells  us  what  must  fol- 
low from  this  similarity:  "  The  nature,  the  number,  the  ar- 
rangement, and  the  distance  of  the  atoms  from  one  another  are 
the  same."  In  this  case  what  becomes  of  the  clear  definition 
of  chemical  species,  so  important  for  the  time,  which  was  given 
us  by  Chevreul  in  the  year  1823?  "  The  species  of  compound 
bodies  are  identical  when  the  nature,  the  proportion,  and  the 

11 


MEMOIRS     ON 

arrangement  of  the  atoms  are  the  same,"  In  short,  this  com- 
munication of  Mitscherlich's  remained  in  my  mind  as  the  cinef 
difficulty  in  the  way  of  this  conception  of  bodies. 

It  will  now  be  understood  by  all  that  I  thought  of  this  com- 
munication of  Mitscherlich's  in  the  year  1844  when,  for  the 
reasons  given  above,  I  was  considering  a  possible  connection 
between  the  hemihedrism  of  the  tartrates  and  their  optical 
activity.  At  once  I  thought  that  Mitscherlich  had  erred  in 
one  point.  Apparently  he  had  not  observed  that  his  double 
salt  of  tartaric  acid  is  hemihedral,  while  the  racemic  acid  salt 
is  not.  la  case  this  were  the  fact  his  results  would  have  no 
more  importance;  moreover,  it  would  furnish  the  best  kind  of 
support  for  my  hypothesis  concerning  the  relation  between 
hemihedrism  and  optical  activity. 

I  undertook,  therefore,  to  study  again  the  crystalline  form 
of  both  of  Mitscherlich's  salts.  I  found  that  the  tartaric  acid 
salt,  as  in  the  case  of  all  other  tartrates  which  I  had  examined, 
was  hemihedral;  but  unexpectedly  the  raeemic  acid  salt  was 
also  hemihedral.  Only,  the  hemihedral  faces  in  the  tartrates 
all  lay  in  the  same  direction,  in  the  racemates  some  lay  toward 
the  right  and  some  toward  the  left. 

In  spite  of  the  completely  unexpected  character  of  these  re- 
sults I,  none  the  less,  followed  my  idea.  I  separated  carefully 
the  right  hemihedral  crystals  from  the  left,  and  observed  the 
solution  of  both,  each  by  itself,  in  the  polariscope.  There  I 
saw,  with  as  much  surprise  as  joy,  that  the  right  hemihednil 
crystals  turned  the  plane  of  polarization  toward  the  right,  and 
the  left  hemihedral  crystals  turned  it  toward  the  left;  and  that 
when  I  took  an  equal  mass  of  both  crystals  their  mixed  .solu- 
tions remained  inactive  toward  light  through  the  mutual  com- 
pensation of  the  two  equal  but  opposite  rotations.  I  proceed 
thus  with  the  racemic  acid:  I  obtain  the  sodium-ammonium 
double  salt  in  the  ordinary  way,  and  from  this  solution  crystals 
separate  after  some  days,  which  possess  so  exactly  the  s-arne 
angle  and  the  same  appearance  that  the  celebrated  crystallog- 
rapher  Mitscherlich,  in  spite  of  his  exact  and  carefully  carried 
out  measurements,  did  not  discover  the  slightest  difference  be- 
tween them. 

Nevertheless,  the   arrangement  of  the  molecule  in  the  two 

12 


STEREO-CHEMISTRY. 

salts  is  entirely  different.     This  is  shown  both  by  the  optical 
activity  and  the  asymmetry  of  the  crystals. 

Both  kinds  of  crystals  are  isomorphous,  and  isomorphous 
with  the  corresponding  tartrates.  Yet  the  isomorphism  shows 
a  characteristic  never  before  observed.  It  is  the  isomorphism 
of  two  asymmetric  crystals,  of  an  object  to  its  reflected  image. 
This  comparison  expresses  the  phenomena  in  a  very  satisfactory 
way.  In  fact,  if  I  think  of  the  hemihedral  faces  of  each  of  the 
kinds  of  crystals  as  being  extended  until  they  meet,  I  obtain 
two  asymmetric  tetrahedrons  which  cannot  be  brought  into 
corresponding  positions  in  spite  of  the  identity  of  their  cor- 
responding parts.  Thereupon  I  concluded  that  by  the  crystalli- 
zation of  the  sodium-ammonium  double  salt  of  racemic  acid  I 
had  separated  the  racemic  acid  into  two  asymmetric,  iso- 
morphous groups,  which  were  united  with  one  another  in  the 
racemic  acid.  Nothing  is  simpler  than  to  prove  that  the  two 
kinds  of  crystals  represent  two  different  salts,  from  which  the 
different  acids  can  be  prepared.  One  proceeds  as  in  all  such 
cases.  Convert  each  salt  into  the  lead  or  barium  salt  and  from 
this  isolate  the  free  acid  by  means  of  sulphuric  acid.  The 
study  of  these  acids  possesses  an  extraordinary  interest;  I  know 
of  none  greater.  Yet  before  I  describe  it  allow  me  to  interpo- 
late some  reminiscences  in  regard  to  their  discovery. 

V. 

THE  publication  of  these  facts  brought  me  naturally  into 
communication  with  Monsieur  Biot,  who  entertained  no  doubts 
as  to  their  correctness.  Since  he  wished,  however,  to  discuss 
them  before  the  Academy,  he  invited  me  to  come  and  repeat 
the  different  experiments  before  him.  He  gave  me  some 
racemic  acid  which  he  himself  had  previously  examined  and 
found  to  be  entirely  inactive  toward  polarized  light.  I  pre- 
pared from  it,  in  his  presence,  the  sodium-ammonium  double 
salt,  the  sodium  hydroxide  and  ammonia  for  which  he  also 
wished  to  furnish.  The  solution  was  then  placed  in  his  labora- 
tory and  allowed. slowly  to  evaporate  ;  when  30  to  40  grams  of 
the  crystals  had  separated,  he  again  called  me  to  the  College 
de  France  to  collect  and  distinguish  by  their  crystallographic 
character  the  right  and  left  rotating  crystals  from  one  another, 

13 


MEMOIRS     ON 

under  his  direct  observation;  he  bade  me  repeat  the  declaration 
that  the  crystals  which  I  placed  at  his  right  hand  would  rotate 
the  plane  of  polarization  to  the  right  and  the  others  would  ro- 
tate it  to  the  left. 

After  this  had  been  done,  he  declared  that  he  himself  would 
complete  the  experiments.  He  prepared  the  carefully  weighed 
solutions  and,  when  he  was  ready  to  make  the  observations  in 
the  polarizing  apparatus,  he  called  me  again  into  his  laboratory. 
He  first  put  into  the  apparatus  the  most  interesting  solution, 
the  one  which  should  rotate  toward  the  left.  Without  making 
a  reading,  but  upon  the  instant,  he  noted  a  change  of  color  in 
the  two  halves  of  the  field  of  vision,  he  recognized  an  important 
laevorotation. 

Then  the  excited  old  man  seized  my  hand  and  said:  "My 
dear  child,  I  have  all  my  life  so  loved  this  science  that  I  can 
hear  my  heart  beat  for  joy." 

Gentlemen,  you  will  pardon  these  personal  reminiscences, 
which  will  never  fade  from  my  memory.  Following  the 
custom  of  our  times  one  would  avoid  such  things  in  a  scientific 
discussion;  but  it  appears  to  me  that  perhaps  such  reminiscences 
are  of  sufficient  biographical  interest,  that  information  of  this 
nature  may  be  brought  before  the  chemical  society  in  an  oral 
communication.  However,  there  is  here  more  than  personal 
reminiscences.  Much  to  the  satisfaction  of  scientists  there 
came  to  M.  Biot  the  great  satisfaction  of  seeing  his  assumptions 
verified.  For  more  than  twenty  years  M.  Biot  had  endeavored 
in  vain  to  bring  chemists  to  his  views  that  the  study  of  the 
rotation  phenomena  was  one  of  the  surest  means  of  advancing 
in  the  investigations  of  the  molecular  constitution  of  bodies. 

VI. 

LET  us  return  to  the  two  acids  which  are  formed  from  the 
two  kinds  of  crystals  produced  when  the  sodium-ammonium 
double  salt  is  allowed  to  evaporate.  I  have  already  said  that 
nothing  is  more  interesting  than  the  study  of  these  acids.  In 
fact,  the  acid,  which  is  obtained  from  the  right  hemihedral 
double  salt,  rotates  the  plane  of  polarization  to  the  right  and  is 
identical  with  ordinary  tartaric  acid. 

The  other  rotates  toward  the  left  as  does  the  salt  from  which 

14 


STEREO-CHEMISTEY. 

it  is  obtained.  The  amount  of  the  rotation  of  the  plane  of 
polarization  is  exactly  the"  same  with  both  acids.  The  dextro 
acid  follows  certain  laws  in  its  rotation  which  are  not  found 
with  any  other  active  substance. 

The  laevo  acid  shows  the  same  laws  in  an  opposite  sense,  not 
the  slightest  difference  has  been  observed. 

Here  is  the  proof  that  racemic  acid  is  a  compound  of  both 
acids,  equivalent  with  equivalent.  If  one  mixes  concentrated 
solutions  of  the  two  containing  equal  masses,  as  I  do  now 
before  you,  a  compound  is  formed  with  the  development  of 
heat.  The  solution  partly  solidifies  with  abundant  crystalliza- 
tion of  racemic  acid,  which  is  identical  with  the  natural 
racemic  acid.  All  of  the  chemical  and  crystallographic  prop- 
erties of  one  acid  are,  under  the  same  conditions,  shown  in  the 
other;  and  in  all  cases  identical  products  are  obtained,  except 
that  they  cannot  be  brought  into  corresponding  positions  ; 
they  are  products  which  are  to  one  another  as  the  right  hand 
is  to  the  left  hand.  They  have  the  same  forms,  the  same  faces, 
the  same  angles,  and  in  both  cases  are  hemihedral. 

The  only  difference  between  them  is  the  direction  of  the 
hemihedral  faces  toward  the  right  in  one  case;  and  toward  the 
left  in  the  other,  and  in  the  direction  of  their  optical  rotation. 

VII. 

IT  is  clear  from  all  of  these  facts  that  we  have  to  do  with  two 
isomeric  bodies  whose  general  character  of  molecular  similarity 
and  dissimilarity  we  know.  Do  you  remember  the  definition 
of  chemical  species  which  I  have  given  above  ?  A  species  is 
composed  of  all  of  the  individuals  which  contain  the  same 
elements,  in  the  same  proportions,  and  with  the  same  arrange- 
ment. All  the  properties  of  bodies  are  functions  of  these 
factors  and  the  object  of  all  our  investigations  is,  through  a 
study  of  these  properties,  to  gain  a  knowledge  of  these  factors. 
In  the  case  of  isomeric  bodies  the  elements  and  their  propor- 
tion are  the  same,  the  arrangement  of  the  atoms  alone  is  differ- 
ent. The  greatest  interest  in  isomerism  comes  from  its  intro- 
duction of  the  idea  that  bodies  may  be  entirely  changed  in 
character  by  a  rearrangement  of  the  atoms  in  the  chemical 
molecule,  this  is  actually  the  case.  Still  there  are  no  isomeric 

15 


MEMOIRS     ON 

bodies  in  which  we  know  the  actual  relation  of  the  molecular 
arrangements  to  one  another.  This  deficiency  will  be  made 
good  for  the  first  time  by  the  discovery  of  the  constitution  of 
racemic  acid  and  the  mutual  relation  between  the  dextro-  and 
Isevotartaric  acids.  We  know,  in  fact,  on  the  one  hand,  that 
the  molecular  arrangement  of  both  tartaric  acids  is  asymmetric; 
on  the  other,  that  they  are  entirely  the  same,  with  the  ex- 
ception that  the  asymmetry  is  shown  in  opposite  senses.  Are 
the  atoms  of  the  dextro  acid  arranged  in  the  form  of  a  right- 
handed  spiral,  or  are  they  situated  at  the  corners  of  an  irregular 
tetrahedron,  or  do  they  have  some  other  asymmetric  grouping? 
This  we  do  not  know.  But  without  doubt  the  atoms  possess 
an  asymmetric  arrangement  like  that  of  an  object  and  its  re- 
flected image. 

Quite  as  certain  is  it  that  the  atoms  of  the  laevo  acid  possess 
exactly  the  opposite  grouping. 

Finally  we  know  that  racemic  acid  arises  from  the  union  of 
two  asymmetric  groups  whose  atoms  are  arranged  in  inverse 
order.  From  now  on,  the  knowledge  of  the  chemical  and 
physical  similarities  and  differences  corresponding  to  this  kind 
of  grouping,  the-  relationships  between  which  we  know,  offers 
especial  interest  and  gives  a  firm  foundation  to  molecular  me- 
chanics. It  permits  us  to  determine  the  relation  between  the 
physical  and  chemical  properties  and  the  molecular  arrange- 
ment which  is  the  cause  of  these  properties  ;  or  conversely,  it 
enables  us  from  the  properties  to  infer  their  first  cause. 

The  general  relations  between  the  properties  and  the  cor- 
responding arrangement  may  be  summed  up  as  follows  : 

(1)  If   the   elementary  atoms  of  an  organic  substance   are 
asymmetrically   grouped,    the   crystal   form   shows   molecular 
asymmetry  by  nonsuperposable  hemihedrism. 

Thus  is  the  cause  of  hemihedrism  recognized. 

(2)  The  existence  of  this  molecular  asymmetry  shows  itself 
farther  by  optical   rotation.     Thus  is  seen  the  cause  of  this 
optical  activity.* 

*  Fresnel,  with  that  clearness  of  insight  which  he  so  frequently  showed, 
saw  to  a  certain  extent  the  cause  of  optical  activity.  He  expressed  it  in 
an  article  in  1823  in  volume  28  of  the  Annales  de  Chimie  et  de  Physique, 
as  follows:  "  Quartz  crystals  show  optical  phenomena  which  are  not  con- 
sistent with  complete  parallelism  of  the  molecular  lines,  and  it  appears 
to  indicate  a  regular  and  progressive  deviation  of  these  lines." 

16 


STEREO-CHEMISTKY. 

(3)  When  two  forms  with  nonsuperposable  asymmetric  mole- 
cules arise,  as  in  the  case  of  dextro-  and  laevotartaric  acid  and 
all  of  their  derivatives,  the  chemical  properties  of  these  identi- 
cal but  optically  opposite  bodies  are  the  same;  whence  it  follows 
that  this  kind  of  oppbsiteness  of  position  and  similarity  does 
not  affect  the  ordinary  action  of  chemical  affinity.  I  am  mis- 
taken in  regard  to  this  latter  point.  I  must  make  a  limita- 
tion, an  important  and  very  instructive  limitation. 

But  the  time  is  lacking  to-day  to  develop  it  with  the  fullness 
it  deserves.  It  will  therefore  be  treated  in  the  next  lecture. 


SECOND  LECTURE. 
I. 

GENTLEMEN":  When  one  investigates  bodies  with  regard  to 
their  form  and  to  the  recurrence  of  their  identical  parts  he  soon 
recognizes  that  they  may  be  divided  into  two  great  classes  with 
the  following  characteristics:  One  class  is  mad"  up  of  forms 
the  reflected  image  of  which  can  be  superpose  1  upon  the 
original;  with  the  forms  of  the  other  class  this  cannot  be  done, 
although  the  forms  are  the  same  in  all  their  parts.  A  straight 
stairway,  a  twig  with  oppositely  set  leaves,  a  cube,  the  human 
body  are  examples  of  the  first  class  of  bodies,  A  winding  stair- 
way, a  twig  with  leaves  set  in  the  form  of  a  spiral,  a  screw,  a 
hand,  an  irregular  tetrahedron  are  forms  of  the  second  class. 
The  forms  of  the  second  class  have  no  plane  of  symmetry. 

On  the  other  hand,  we  know  that  compound  bodies  are 
aggregates  of  identical  molecules,  and  that  the  latter  them- 
selves consist  of  a  collection  of  elementary  atoms,  adjusted 
according  to  laws  which  are  determined  by  their  nature,  their 
proportion,  and  their  arrangement.  The  characteristics  of 
every  compound  body  are  inherent  in  its  chemical  molecule, 
and  this  is  a  group  of  atoms  which  do  not  move  about  freely 
among  themselves,  but  which  stand  in  a  definite  relation  to 
one  another.  This  is  the  belief  of  all  physicists  in  regard  to 
the  constitution  of  matter. 

This  once  recognized,  it  certainly  would  be  very  remarkable, 
if  nature,  which  in  its  results  shows  such  manifold  variety  and 

17 


MEMOIRS     ON 

whose  laws  permit  the  existence  of  so  many  kinds  of  bodies,  did 
not,  in  complex  molecules,  give  us  atom  groups  belonging  to 
the  two  categories  into  which  material  things  may  be  divided. 
It  would,  in  other  words,  be  astonishing  if  among  all  chemical 
substances,  natural  and  artificial,  there  were  not  individuals 
which  were  superposable  with  their  reflected  image,  'and  others 
which  were  not  superposable  with  their  reflected  image.  It 
appears  to  be  a  fact,  as  was  foreseen,  that  all  chemical  com- 
pounds without  exception  fall  into  two  classes,  one  in  which  the 
reflected  image  is  superposable  with  the  original  and  the  other 
in  which  it  is  not  superposable. 

II. 

IT  is  very  easy  to  prove  that  this  is  a  logical  consequence 
which  must  follow  from  statements  in  my  first  lecture. 

In  order  to  make  it  entirely  clear  I  shall  briefly  repeat  the 
main  features  of  the  conclusive  reasons  with  which  I  closed  the 
previous  chapter.  1  prepare,  with  the  help  of  the  natural 
racemic  acid,  £he  sodium-ammonium  double  salt. 

It  separates  in  beautiful  crystals.  If  one  observes  the  solu- 
tion of  a  mass  of  this  double  salt  in  a  polarizing  apparatus,  no 
sign  of  optical  activity  is  detected;  if  the  acid  is  set  free  from 
the  salt,  racemic  acid  is  again  obtained,  identical  with  that  from 
which  the  salt  is  first  prepared. 

Thus  far  everything  is  perfectly  simple  and  natural,  and  one 
might  believe  that  he  was  dealing  with  the  crystallization  of  an 
ordinary  salt.  But  this  is  not  the  case. 

If  you  will  take  another  portion  of  the  same  crystals  and  ex- 
amine the  individual  crystals,  you  will  find  that  half  of  them 
have  the  form  of  the  model  which  I  now  show  you  and  which 
is  characterized  by  nonsuperposable  hemihedrism.  The  other 
half  possess  the  opposite  form  identical  with  the  first  in  all 
their  respective  parts,  but,  in  spite  of  this,  nonsuperposable 
with  it.  If  the  two  kinds  of  crystals  are  separated,  and  each 
kind  dissolved  by  itself,  it  is  found  that  one  of  the  solutions 
turns  the  plane  of  polarization  toward  the  right,  and  the  other 
turns  it  toward  the  left,  and  both  through  equal  angles. 

Finally  if  the  acids  are  prepared  from  these  two  kinds  of 
crystals  by  the  ordinary  chemical  means,  it  is  seen  that  one  is 

18 


S  T  E  K  E  0  -  C  H  E  M I  S  T  K  Y . 

identical  with  ordinary  tartaric  acid,  and  that  the  other  is  in 
most  respects  the  same  without  being  identical  with  it.  They 
stand  to  one  another  in  the  same  relations  as  the  salts  from 
which  they  are  prepared. 

They  resemble  one  another  as  the  right  hand  resembles  the 
left ;  or  better  still  as  two  irregular  asymmetric  tetrahedrons  ; 
and  the  same  analogies  and  the  same  differences  recur  in  all  of 
their  derivatives.  Whatever  can  be  done  with  one  can  also, 
under  the  same  conditions,  be  done  with  the  other,  arid  the 
products  thus  formed  have  always  the  same  properties  with  the 
single  difference  that  one  rotates  the  plane  of  polarization  to- 
ward the  right,  while  the  other  rotates  it  toward  the  left  ;  and 
that  the  forms  of  the  corresponding  substances,  although  in  all 
particulars  identical,  are  not  superposable. 

All  of  these  clearly  demonstrated  facts  lead  us  to  ascribe  the 
general  outer  characteristics  of  these  acids  and  their  compounds 
to  their  individual  chemical  molecules. 

To  conclude  otherwise  would  be  to  bid  defiance  to  the  clearest 
rules  of  logic. 

Thus  we  reach  the  following  conclusions  : 

(1)  The  molecule  of  tartaric  acid,  however  else  it  may  be 
constituted,  is  asymmetric,  and   has  that  kind  of  asymmetry 
which  is  not  superposable  with  its  mirrored  image. 

(2)  The  molecule   of  the   lasvotartaric   acid   is   formed   by 
exactly  the  opposite  grouping  of  the  atoms. 

By  what  properties  do  we  recognize  molecular  asymmetry  ? 

On  the  one  hand  by  the  occurrence  of  nonsuperposable  hemi- 
hedral  forms,  and  on  the  other,  chiefly,  by  optical  activity  as 
soon  as  the  body  is  brought  into  solution. 

Let  us  now,  this  principle  being  assumed,  examine  all  natural 
and  artificial  bodies,  we  shall  easily  find  that  a  number  of  them 
possess  both  hemihedral  forms  and  molecular  rotatory  power,  and 
that  all  others  show  neither  the  one  nor  the  other  of  these  proper- 
ties. I  am  therefore  justified  in  saying  that  the  sufficient  and 
necessary  consequence  of  the  facts  given  in  my  first  lecture  can  be 
stated  as  follows  :  All  bodies  (I  use  the  term  here  in  its  chem- 
ical sense)  are  divided  into  two  great  classes, — into  bodies  which 
are  superposable  with  their  reflected  image, — and  into  those  in 
which  the  reflected  image  is  not  superposable  with  the  original  : 

19 


MEMOIRS     OX 

into   bodies  with  symmetrical  atom  grouping,  and  into  those 
with  asymmetrical  atom  grouping. 

III. 

HERE  we  meet  with  a  phenomenon  that  is  well  calculated  to 
excite  our  attention,  even  when  it  is  considered  by  itself  in- 
dependently of  the  whole  of  the  conclusions  which  come  later  : 
All  artificial  bodies  and  all  minerals  have  superposable  images. 
Opposed  to  these  are  many  organic  substances  (I  might  say 
nearly  all,  if  I  were  to  specify  only  those  which  play  an  im- 
portant role  in  plant  and  animal  life)  all  of  which  are  important 
substances  to  life,  are  asymmetric,  and  indeed  have  the  kind  of 
asymmetry  in  which  the  image  is  not  superposable  with  the 
object. 

Before  going  farther  I  shall  answer  some  objections  which 
must  have  presented  themselves  to  you. 

IV. 

THERE  is,  for  example,  quartz  you  will  say.  We  have  heard 
in  the  first  lecture  that  quartz  shows  both  kinds  of  asymmetry. 
Its  hemihedral  crystal  form  was  observed  by  Hauy,  and  its  opti- 
cal activity  was  discovered  by  Arago. 

In  spite  of  this,  however,  it  lacks  molecular  asymmetry. 

In  order  to  understand  this  we  must  proceed  a  little  farther 
in  the  knowledge  of  the  phenomena  which  we  are  considering. 
"We  shall  find  thereby  an  explanation  of  the  similarities  and  the 
differences,  which  we  have  already  noted,  between  quartz  and 
the  natural  organic  products. 

Permit  me,  in  a  crude  but  fundamentally  correct  way,  to  il- 
lustrate to  you  the  structure  of  quartz  and  the  natural  organic 
products.  Consider  a  spiral  staircase  the  steps  of  which  are 
cubes  or  other  objects  superposable  with  their  reflected  image. 
If  you  destroy  the  staircase  the  asymmetry  disappears.  The 
asymmetry  of  the  staircase  was  due  to  the  method  of  placing 
together  the  single  steps.  So  it  is  with  quartz.  The  quartz 
crystal  is  the  completed  staircase.  It  is  hemihedral,  and  in 
consequence  it  acts  upon  polarized  light.  If,  however,  the 
crystal  is  dissolved,  fused,  or  in.  any  other  way  has  its  physical 
structure  destroyed,  its  asymmetry  is  no  longer  present,  at  the 

20 


STEBEO-OHEMISTBY. 

same  time  all  action  upon  polarized  light  disappears  ;  as  is  the 
case,  for  example,  with  an  alum  solution.  Consider  again 
the  same  spiral  staircase,  the  steps  of  which  are  formed  by  ir- 
regular tetrahedrons.  You  may  destroy  the  staircase,  but  the 
asymmetry  remains,  because  you  have  to  deal  with  a  collection 
of  irregular  tetrahedrons. 

They  may  have  any  sort  of  arrangement,  but  each  one  of 
them  has  its  own  asymmetry.  Thus  it  is  with  organic  bodies 
where  every  molecule  has  its  own  asymmetry  which  finds  ex- 
pression in  the  crystal  form.  When  the  crystals  are  destroyed 
by  solution  there  results  a  fluid  which  is  active  toward  polar- 
ized light,  because  it  consists  of  molecules,  which  have,  indeed, 
no  fixed  position  with  regard  to  one  another,  but  every  one  of 
which  has  the  same  asymmetry. 

V. 

QUARTZ  is  thus  not  molecularly  asymmetric,  and  up  to  the 
present  we  do  not  have  any  mineral  which  possesses  molecular 
asymmetry.  I  have  said  that  this  statement  must  extend  to 
artificial  bodies  which  are  prepared  in  the  laboratory.  In  re- 
gard to  this  point  some  farther  consideration  is  necessary.  One 
can,  it  may  be  objected,  artificially  convert  the  natural  cam- 
phor which  is  asymmetric,  into  camphoric  acid  which  is  also 
asymmetric  ;  aspartic  acid  is  produced  by  chemical  processes 
from  asparagine  and  is  asymmetric,  as  is  also  the  asparagine 
from  which  it  is  prepared,  and  I  could  mention  many  other 
similar  cases.  Yet  no  one  doubts  that  camphoric  acid  and 
aspartic  acid  owe  their  asymmetry  to  camphor  and  asparagine. 
It  was  present  in  the  mother  substances  and  is  from  these 
passed  over  to  their  derivatives,  more  or  less  changed  by  sub- 
stitution. 

There  is  no  better  proof  of  the  stability  of  a  primary  type, 
than  the  continuation  of  the  optical  properties,  in  a  group  of 
derivatives  which  are  related  to  one  another  by  having  a  com- 
mon origin.  When  I  assert  that  no  artificial  substances  with 
molecular  asymmetry  are  known,  I  speak  of  artificial  substances 
in  the  proper  sense  of  the  word,  which  are  formed  in  all  their 
parts  from  the  elements,  or  are  produced  from  bodies  which 
are  not  asymmetric.  Alcohol,  for  example,  is  not  asymmetric, 

21 


MEMOIRS     ON 

its  molecule  would,  could  we  isolate  it  and  look  at  it  in  a  mir- 
ror, give  an  image  which  could  be  superposed  upon  it.  Not  a 
derivative  of  alcohol  is  asymmetric.  I  could  vastly  increase 
the  number  of  examples  of  this  kind.  It  goes  even  farther;  if 
you  take  a  body  which  is  asymmetric,  you  may  be  sure  that  the 
asymmetry  will  disappear  as  soon  as  it  is  subjected  to  an  ener- 
getic chemical  reaction.  Thus  tartaric  acid  is  asymmetric;  pyro- 
tartaric  acid  is  no  longer  asymmetric.  Malic  acid  is  asymmetric; 
but  the  maleic  and  fumaric  acids  discovered  by  Pelouze  are  not 
asymmetric.  Gum  is  asymmetric;  but  mucic  acid  is  not  asym- 
metric. Artificial  substances  have  thus  no  molecular  asymme- 
try, and  I  know  of  no  more  thorough  going  difference  than  just 
this  between  the  substances  which  are  formed  under  the  influ- 
ence of  life  and  other  substances. 

Let  us  tarry  here  a  little,  you  will  see  that  in  the  course  of 
these  lectures,  the  physiological  side  of  these  studies  comes 
more  and  more  into  prominence. 

We  will  repeat  the  chief  classes  of  natural  organic  substances: 

Cellulose,  starch,  gums,  sugar,  .  .  .  tartaric  acid,  malic  acid, 
quinic  acid,  tannic  acid,  .  .  .'  morphine,  codeine,  quinine, 
strychnine,  brucine,  .  .  .  turpentine  oil,  citron  oil,  .  .  .  albu- 
men, fibrine,  gelatine.  All  of  these  naturally  occurring  bodies 
have  molecular  asymmetry.  All  solutions  of  these  bodies  pos- 
sess the  power  of  rotating  the  plane  of  polarization,  a  necessary 
and  all  sufficient  characteristic  for  determining  their  asym- 
metry, even  when,  owing  to  their  noncrystalline  character, 
their  hemihedral  characteristics  are  lacking. 

There  are  many  natural  substances  which  are  not  asymmetric. 

But  can  we  call  them  natural  with  the  same  right  as  the 
others  ?  Must  we  not,  in  such  substances  as  oxalic  acid, 
salicylic  aldehyde,  fumaric  acid,  etc.,  which  arise  from  such 
reactions  as  can  be  carried  out  in  the  laboratory,  see  derivatives 
of  the  real  natural  substances?  These  products  appear  to  me 
to  be  the  same  for  the  plant  organisms  as  urea,  uric  acid, 
creatine,  glycine,  are  for  the  animal  organism;  much  more 
decomposition  products,  than  transition  products,  if  I  may  so 
express  it.  It  is  very  interesting  to  follow  this  point  of  view 
experimentally. 

We   must  still  mention  that  a  number  of  apparently  sym- 


STEREO-CHEMISTKY. 

metrical  bodies  have  properties  like  that  of  racemic  acid. 
There  is  yet  lacking  in  the  chemical  nomenclature  a  word  to 
express  the  fact  that  through  the  compensation  of  two  opposite 
asymmetries  a  double  asymmetry  may  result  which  is  itself 
symmetrical. 

The  knowledge  that  ordinary  asymmetry  is  a  direct  organic 
principle,  and  the  knowledge  of  the  lack  of  this  property  in  all 
bodies  of  dead  nature,  permits  us  to  extend  and  render  more  ex- 
act our  assumptions  in  regard  to  this  noteworthy  molecular 
property. 

VI. 

IN  the  year  1850  M.  Dessaignes,  whose  inventive  skill  is 
known  to  all  chemists,  informed  the  Academy  that  he  had  suc- 
ceeded in  converting  the  acid  ammonium  salt  of  malic  acid  into 
aspartic  acid.  This  step  confirmed  the  important  results  which 
had  been  obtained  by  Piria  some  years  earlier. 

M.  Piria  had  succeeded  in  converting  asparagine  and  aspartic 
acid  into  malic  acid.  Dessaignes  now  showed  that  the  as- 
partic acid  could  be  regenerated  from  the  malic  acid.  Thus 
far  all  of  Dessaignes'  observations  were  entirely  in  accordance 
with  our  experience  even  from  an  optical  standpoint.  For  I 
had  observed  that  asparagine,  aspartic  acid,  and  malic  acid 
were  all  active  toward  polarized  light. 

The  chemical  transformation  of  the  one  body  into  the  other 
was  not  surprising. 

Some  months  later  Dessaignes  went  a  step  farther. 

He  announced  that,  not  only  the  acid  ammonium  malate, 
but  also  the  ammonium  salt  of  furnaric  acid  and  maleic  acid 
have  the  property  of  changing  to  aspartic  acid  when  warmed. 
In  this  I  saw  an  improbability.  For,  if  it  were  as  Dessaignes 
had  stated,  he  had  made  a  discovery  the  importance  of  which 
he  had  never  dreamed.  I  had,  in  fact,  observed  that  fumaric 
acid,  maleic  acid,  and  all  of  their  salts  were  inactive  toward 
polarized  light. 

Had  M.  Dessaignes  actually  converted  their  ammonium  salts 
into  aspartic  acid,  he  had,  for  the  first  time,  prepared  an 
asymmetric  compound  from  a  nonasymmetric  body.  To  me  it 
appeared  more  probable  that  the  aspartic  acid  of  Dessaignes 

23 


MEMOIRS     ON 

differed  from  the  natural  aspartic  acid  by  its  lack  of  molecular 
rotatory  power.  Dessaignes  had,  indeed,  carefully  compared 
the  properties  of  the  artificial  acid  with  the  natural  acid  and 
had,  as  he  said,  found  them  identical. 

But  I  knew  better  than  any  one  else,  through  my  experience 
with  Mitscherlich's  statement,  which  I  have  spoken  of  in  the 
first  lecture,  the  delicate  character  of  the  proof  of  the  identity 
of  chemical  bodies  in  which  the  greatest  similarity  of  proper- 
ties may  conceal  a  fundamental  difference. 

I  did  not  hesitate,  therefore,  to  believe  that  the  new  facts 
announced  by  Dessaignes  needed  confirmation.  The  clearing 
up  of  this  point  meant  so  much  for  the  results  which  I  have 
had  the  honor  to  bring  before  you  that  I  went  at  once  to 
Vendome  and  laid  my  views  before  M.  Dessaignes. 

He  handed  me  at  once  a  sample  of  his  acid.  Upon  my  re- 
turn to  Paris  I  actually  recognized  that  the  acid  of  M.  Des- 
saignes' was  only  an  isomer  of  the  natural  aspartic  acid,  that 
is  to  say,  the  acid  which  is  obtained  from  asparagine,  and  it 
differed  from  this,  as  I  had  foreseen,  in  optical  rotatory  power, 
which  was  entirely  absent  in  the  artificial  acid,  but  present  in 
the  natural  acid.  But  in  all  other  physical  and  chemical  prop- 
erties they  showed  the  greatest  analogy,  so  great  that  Des- 
saignes, not  being  placed  upon  his  guard  by  preconceived  ideas, 
concluded  that  the  two  substances  were  actually  identical. 

That  which  interested  me  the  most  in  the  study  of  this  new 
compound  (which  did  not  furnish  any  conspicuous  crystalline 
derivatives)  was  its  transformation  into  malic  acid.  It  was 
known  that  Piria,  as  I  have  just  mentioned,  had  some  time  be- 
fore given  the  means  of  transforming  asparagine  and  aspartic 
acid  into  malic  acid,  and  I  had  convinced  myself  by  the  most 
careful  investigations  that  this  malic  acid  was  identical  with 
that  obtained  from  the  berries  of  the  mountain  ash,  apples, 
grapes,  and  tobacco. 

I  used,  therefore,  this  process  of  Piria/s  and  thereby  con- 
verted this  new  acid  into  a  new  malic  acid  so  similar  to  the 
natural  malic  acid  that  a  chemist  would  have  found  the  great- 
est difficulty  in  distinguishing  between  them,  had  he  not 
known  beforehand  what  the  real  difference  was,  this  malic  acid 
had  no  action  whatever  upon  polarized  light  and  all  of  its  salts 
were  the  same  in  this  respect. 

24 


STEREO-CHEMISTRY. 

By  the  comparison  of  certain  derivatives  of  these  two  malic 
acids  the  relative  molecular  arrangement  in  these  curious  isomers 
is  not  apparent,  but  with  other  derivatives  it  is  entirely  evident. 
If  we  consider,  for  example,  the  ordinary  acid  calcium  salt  of 
malic  acid,  and  the  corresponding  inactive  compounds,  we 
find  their  composition  just  the  same,  and  their  crystal  form  is 
almost  the  same,  with  the  difference  that  the  active  form  shows 
four  small  hemihedral  faces  which  are  lacking  in  the  inactive 
form.  Whence  it  follows  that  the  active  crystal  and  its  re- 
flected image  are  enantimorphous,  while  the  image  of  the  in- 
active crystal  is  identical  with  the  original.  In  regard  to  all 
points  which  do  not  have  to  do  with  hemihedrism  the  two 
forms  are  completely  alike. 

Who  can  have  any  doubt  concerning  the  relative  molecular 
arrangement  of  these  two  salts  ?  Is  it  not  clear  that  we  here 
have  to  deal  with  a  malic  acid  identical  with  the  natural  acid, 
except  that  it  has  had  its  asymmetry  suppressed  ? 

It  is  untwisted  natural  malic  acid  if  I  may  so  express  it.  If 
the  natural  acid  as  regards  its  molecular  arrangement  be  com- 
pared to  a  spiral  staircase,  then  the  artificial  acid  is  the  same 
staircase  made  np  of  identical  steps  but  straight,  instead  of 
spiral.  It  might  be  asked  whether  the  new  malic  acid  was  not 
the  racemic  form  of  this  group,  that  is,  the  compound  of  the 
right  and  left  malic  acids. 

This  has  very  slight  probability,  for  in  this  case  not  only 
would  one  active  body  be  made  from  an  inactive  substance,  but 
two  active  bodies  would  be  produced,  a  dextro-  and  laevorota- 
tory  substance. 

However,  I  have  recognized  that  just  as  there  is  an  inactive 
nonasym metric  malic  acid,  there  exists  a  nonasym metric  in- 
active tartaric  acid  which  is  very  different  fromracemio  acid  as 
it  cannot  be  split  up  into  a  dextro-  and  laevo-tartaric  acid.  It  can 
no  longer  be  doubted,  therefore,  that  here  we  have  to  deal  with 
either  dextro-  or  laevo-tartaric  acid  which  has  become  nonasym- 
metric. 

I  have  also  discovered  inactive  amyl  alcohol  from  which  a 
whole  series  of  inactive  products,  corresponding  to  the  group  of 
active  amyl  alcohols,  can  be  derived. 

We  find  ourselves  then,  thanks  to  the  discovery  of  these  in- 

25 


MEMO  IBS    ON 

active  bodies  in  possession  of  a  fruitful  idea.  A  substance  is 
asymmetric  dextro-  or  laevorotatory  but  by  certain  processes, 
which  must  be  sought  for  and  discovered  for  each  case,  isomeric 
changes  may  cause  it  to  lose  its  molecular  asymmetry,  may 
cause  it  to  be  untwisted,  to  make  use  of  a  rough  simile,  and 
cause  its  atoms  to  become  so  arranged  that  they  are  superpos- 
able  with  their  reflected  image.  In  this  way  every  asymmetric 
substance  gives  four  variations  or  better  four  important  sub- 
divisions ;  the  dextro  body,  the  laevo  body,  the  combination  of 
dextro  and  laevo  bodies,  and  the  body  which  is  neither  dextro 
nor  laevo  nor  yet  a  combination  of  the  two. 

VII. 

THE  general  conclusions  drawn  from  the  foregoing  studies 
throw  a  new  light  upon  our  idea  of  molecular  mechanics. 

We  recognize  from  these  that  when  natural  organic  bodies 
arise  under  the  influence  of  vegetable  life  they  are  usually 
asymmetric  in  opposition  to  minerals  and  synthetical  bodies, 
this  arrangement  of  the  elementary  atoms  is  not  essential  to 
the  existence  of  the  molecule  since  the  spiral  organic  group 
can,  so  to  speak,  be  untwisted  and  it  then  assumes  the  general 
character  of  the  artificial  or  mineral  bodies.  It  appears  only 
logical  to  assume  that  the  artificial  bodies  have  the  power  to 
take  on  the  asymmetric  arrangement  of  their  atoms  and  thus 
become  like  the  natural  bodies. 

The  conditions  under  which  such  a  change  can  be  brought 
about  are  yet  to  be  investigated. 

As  a  last  consideration  and  to  connect  the  foregoing  state- 
men  ,  we  can  say  :  the  groups  of  elementary  atoms  which  form 
compound  bodies  may  assume  two  different  conditions  which 
correspond  to  the  two  general  types  to  which  all  material  bodies 
can  be  referred.  The  form  of  the  group  is  identical  with  its 
reflected  image,  or  it  is  enantimorphous  with  it.  But  the  latter 
type  shows  two  forms,  since  both  enantimorphous  forms  are 
produced.  Moreover  there  must  be  added  to  this  the  case  of 
the  union  of  the  two  opposite  types  which  arise  in  pairs  of 
identical  but  enantimorphous  forms,  reminding  one  of  the 
pairs  of  limbs  of  the  higher  animals.  Thus  there  are  for  the 
atom  groups,  of  which  all  matter  consists,  four  well  marked 

26 


STEREO-CHEMISTEY. 

arrangements.  In  all  of  our  investigations,  therefore,  we  must 
aim  to  prepare  each  substance  in  its  several  varieties. 

All  of  these  deductions  are  so  logically  necessary  that  it  ap- 
pears well-nigh  impossible  to  doubt  them.  How  can  one  avoid, 
for  example,  the  assumption  that  corresponding  to  a  dextro- 
rotatory body  there  must  be  a  laevorotatory  body,  now  that  we 
know  the  cause  of  the  dextro-  and  laevorotatory  character  ? 
That  would  be  to  doubt  that  an  irregular  tetrahedron  had  an 
enantimorphous  image,  or  that  for  a  right-handed  screw  there 
could  be  a  corresponding  left-handed  screw,  or  that  a  right 
hand  was  matched  by  a  left  hand. 

Therefore  the  elementary  constituents  of  all  living  matter 
will  assume  one  or  the  other  of  the  opposite  asymmetries  ac- 
cording as  the  mysterious  life  force,  which  causes  asymmetry  in 
natural  bodies,  acts  in  one  direction  or  another. 

Perhaps  this  will  disclose  a  new  world  to  us.  Who  can  fore- 
see the  organization  that  living  matter  would  assume  if  cellu- 
lose were  laevorotatory  instead  of  being  dextrorotatory,  or  if 
the  Isevorotatory  albumens  of  the  blood  were  to  be  replaced  by 
dextro-rotatory  bodies  ?  These  are  mysteries  which  call  for  an 
immense  amount  of  work  in  the  future,  and  to-day  bespeak 
consideration  in  the  science. 

VIIL 

SINCE  it  has  been  impossible  for  chemistry,  up  to  the  present 
time,  to  prepare  an  asymmetric  body,  one  might  be  led  to  fear 
that  the  method  of  obtaining  the  antipodes  of  the  natural  or- 
ganic bodies  would  always  remain  unknown. 

Fortunately,  this  fear  is  groundless.  For  I  have,  in  fact, 
found  that  by  ordinary  chemical  processes,  as  by  the  action  of 
heat,  a  dextrorotatory  body  may  be  transformed  into  a  laevo- 
rotatory body,  and  vice  versa.  Thus  dextro-tartaric  acid  is 
converted  into  laevo-tartaric  acid,  or  more  properly  into  racemic 
acid,  by  the  action  of  heat  under  certain  conditions  which  it 
would  lead  us  too  far  to  elaborate  here. 

Under  exactly  the  same  conditions  the  laevo-acid  becomes 
dextrorotatory. 

Here   are  10-12   grams   of   entirety  pure  laevo-tartaric  acid 

27 


MEM  OIKS     ON 

which  I  have  obtained  in  this  way.  Its  preparation  has  cost 
me  much  trouble. 

As  M.  Biot  wished  to  investigate  the  dispersion  of  the  laevo- 
tartaric  acid,  he  wished  himself  to  bear  the  high  cost  of  the 
operation,  for  this  conversion  necessitates  the  use  of  cinchon- 
ine  tartrate  and  quinine  tartrate  and  the  base  is  all  lost  because 
the  salt  is  heated  so  high  that  it  is  entirely  destroyed. 

By  this  means  I  have  obtained  enough  racemic  acid  to  yield 
12  grams  of  laevo-tartaric  acid,  which  shows,  in  a  reverse  sense, 
exactly  the  same  optical  properties  as  the  ordinary  tartaric  acid. 

We  must  always  consider  every  analogous  transformation  of 
an  asymmetric  natural  body  into  its  opposite  form  (Antipode) 
as  a  step  in  advance  for  organic  chemistry. 

IX. 

AT  the  close  of  my  first  lecture  I  pointed  out  some  observa- 
tions to  which  we  must  now  devote  some  farther  attention. 
These  observations  relate  to  a  comparison  of  the  chemical  and 
physical  properties  of  optically  isomeric  bodies. 

I  have  already  indicated  the  complete  identity  of  these 
properties,  with  the  exception  of  the  turning  around  of  the 
hemihedral  crystal  faces,  and  the  direction  of  optical  rotation. 
Physical  condition,  crystal  lustre,  solubility,  specific  gravity, 
single  or  double  refraction,  all  of  these  are  not  only  very  simi- 
lar, but  they  are  identical  in  the  strictest  sense  of  the  word. 

This  identity  of  behavior  is  all  the  more  remarkable,  since 
under  conditions  to  be  mentioned  farther  on,  each  variety  may 
be  converted  into  its  exact  opposite. 

We  have  seen  that  all  artificial  or  natural  chemical  com- 
pounds, mineral  or  organic,  must  be  divided  into  two  great 
classes;  symmetrical  compounds  with  identical  reflected  images, 
and  asymmetrical  compounds  with  enantimorphous  reflected 
images.  It  has  been  determined  that  the  identity  of  proper- 
ties in  the  case  of  the  two  tartaric  acids  and  their  derivatives 
persists  so  long  as  they  are  brought  together  with  bodies  of  the 
first  category,  as,  for  example,  potash,  soda,  ammonia,  Jime, 
baryta,  aniline,  alcohol,  the  ethers,  in  short  with  all  bodies 
without  asymmetry,  without  hemihedrism,  without  action  upon 
polarized  light.  On  the  contrary,  if  they  are  subjected  to  the 

28 


STERJEO-CHEMISTRY. 

action  of  bodies  of  the  second  class;  as,  for  example,  asparagine, 
quinine,  strychnine,  brucine,  albumens,  sugars,  etc.,  or  other 
asymmetric  bodies  like  itself,  then  entirely  different  properties 
appear.  The  solubility  is  different.  When  compounds  are 
formed  the  products  differ  in  crystal  form,  in  specific  gravity, 
in  their  amount  of  water  of  crystallization,  in  their  action 
toward  heat,  and  indeed  may  differ  from  one  another  quite  as 
much  as  the  more  distant  isomers. 

Thus  it  appears  that  the  molecular  asymmetry  of  a  body  is 
an  important  agent  for  changing  the  affinity. 

The  two  tartaric  acids  do  not  behave  toward  quinine  as  they 
do  toward  potash,  because  quinine  is  asymmetric  and  potash  is 
not.  Asymmetry  shows  itself  here,  as  before  said,  as  a  property 
which  has  the  power  of  altering  the  chemical  affinity.  I  do  not 
believe  that  any  previous  discovery  has  gone  so  deep  into  the 
mechanics  of  the  problem  of  the  origin  of  chemical  compounds. 

Let  us  attempt  to  make  this  similarity  and  dissimilarity  clear 
by  means  of  an  illustration.  We  may  think  of  a  right-handed 
screw  and  a  left-handed  screw  as  being  driven  into  exactly 
similar,  straight  grained  blocks  of  wood. 

All  of  the  mechanical  conditions  of  the  two  systems  are  the 
same;  this  is  instantly  changed  when  the  same  two  screws  are 
driven  into  a  block  in  which  the  fibres  themselves  have  a  right 
or  left  spiral  arrangement. 

X. 

I  SHALL  interpolate  here  an  extremely  interesting  application 
of  these  facts.  The  recognition  that  the  salts  of  dextro-  and 
laevo-tartaric  acids  assume  very  different  properties  through 
the  rotatory  power  of  the  base,  gives  rise  to  the  hope  that  since 
these  differences  are  caused  by  chemical  affinity,  the  unlike 
affinities  of  the  two  acids  thus  excited  would  give  a  means  of 
splitting  racemic  acid  into  its  constituents.  I  have  made 
many  fruitless  efforts  in  this  direction,  and  I  finally  accom- 
plished my  object  by  the  aid  of  two  new  isorneric  bases,  chini- 
cin  and  cinchonicin,  which  I  easily  prepared  from  quinine  and 
cinchonin.  I  prepared  the  racemic  acid  salt  of  cinchonicin  by 
first  neutralizing  the  base  and  then  adding  as  much  more  acid. 
The  first  crystals  to  separate  were  completely  pure  cinchonicin 


MEMOIRS     ON 

laevo-tartrate.  The  whole  of  the  dextro-tartrate  remained  in 
the  mother-liquor  since  it  is  the  more  soluble.  Little  by  little 
this  also  was  crystallized  out  but  it  had  an  entirely  different 
form  from  the  laevo-tartrate.  One  might  easily  be  lead  to  be- 
lieve that  he  had  to  deal  with  two  entirely  different  salts  of  un- 
equal solubility. 

XI. 

I  SHALL  refer  here  again  to  the  interest  which  attaches  itself 
to  the  different  optical  isomers  on  account  of  this  asymmetric 
force.  It  leads  us  to  ideas  concerning  the  secret  cause  which 
produces  the  asymmetric  arrangement  of  the  atoms  in  organic 
substances.  Whence  comes  this  asymmetry? 

Why  is  one  asymmetry  produced  and  not  the  other?  Go  back 
with  me  to  the  time  at  which  I  recognized  the  absolute  identity 
of  the  physical  and  chemical  properties  of  the  correspond- 
ing right  and  left  rotatory  bodies,  but  still  had  no  idea,  not  a 
suspicion,  of  the  difference  between  these  bodies.  It  was  many 
years  later  that  I  first  recognized  this  difference.  At  that  time 
it  was  to  me  inconceivable  that  nature  should  produce  a  dextro- 
rotatory body  without  its  corresponding  laevo-rotatory  body,  for 
the  same  force,  the  activity  of  which  formed  the  dextro- 
rotatory tartaric  acid  molecule,  as  it  seemed  to  me,  must  also 
produce  thelaevo-rnolecule,  and  thus  racemic  acid  would  result. 

But  why  right  and  left  molecules,  why  not  only  symmetrical 
molecules  like  those  of  the  inorganic  substances  ?  There 
are  certainly  causes  for  this  remarkable  behavior  of  the  molec- 
ular forces,  even  though  it  is  difficult  for  us  to  get  a  clear 
conception  of  them.  I  believe  that  I  am  not  deceived  when 
I  assert  that  we  now  know  one  of  its  most  important  char- 
acteristics. Is  it  not  necessary  and  also  sufficient  to  assume 
that  the  instant  the  plant  organism  arises  an  asymmetric  force 
is  active?  For  we  have  seen  above  that  the  dextro-molecule 
deviates  from  its  laevo-antipode  only  in  those  cases  in  which 
it  is  subjected  to  some  kind  of  an  asymmetric  action.  Do 
such  asymmetric  agencies  arise  from  the  cosmic  influences  light, 
electricity,  magnetism,  heat?  Do  they  perhaps  stand  in  close 
relation  with  the  earth  movements,  with  the  electric  current 
by  means  of  which  physicists  explain  the  earth's  magnetic  pole? 

30 


STEREO-CHEMISTRY. 

We  are  at  present  not  in  a  position  to  offer  the  slightest 
suggestions  in  regard  to  these  points. 

But  I  hold  the  existence  of  an  asymmetric  force  acting  at 
the  origin  of  natural  organic  compounds,  as  proven,  while  this 
force  is  lacking  or  is  without  influence  in  our  synthetical  prep- 
aration of  the  same  compounds,  either  on  account  of  the  more 
rapid  course  of  the  reaction  or  from  some  other  unknown  cause. 

XII. 

WE  now  come  to  the  last  experiment,  which  does  not  fall 
behind  any  of  the  preceding  ones  in  interest,  it  furnishes  us 
evidence  of  the  influence  of  asymmetry  upon  the  life  processes. 
We  have  just  seen  that  asymmetry  modifies  chemical  affinity  ; 
however,  these  results  were  obtained  in  mineral  and  artificial 
reactions,  and  we  know  with  what  caution  the  results  obtained 
in  the  laboratory  can  be  assumed  to  hold  true  in  the  life  proc- 
esses. Therefor  I  have  held  back  the  general  points  of  view 
which  I  wished  to  present  to  you  until  I  had  more  certain 
evidence  that  molecular  asymmetry  had  a  modifying  action 
upon  chemical  affinity,  not  only  in  the  reactions  of  organic 
nature,  but  also  in  those  of  a  physiological  character,  as  in 
fermentation. 

The  remarkable  phenomena  to  which  I  allude  are  as  follows  : 
It  has  been  known  for  a  long  time  through  the  observations  of 
the  German  manufacturers  of  chemical  products  that  the  im- 
pure calcium  tartrate,  so  long  as  it  contained  other  organic 
substances,  underwent  fermentation  upon  standing  during  the 
summer  under  water,  and  thereby  furnished  different  products. 

These  facts  I  took  as  my  starting  point  and  set  up  fermenta- 
tion in  the  ordinary  dextro-ammonium  tartrate  in  the  following 
manner  :  I  took  the  entirely  pure  crystallized  salt  and  dis- 
solved it  and  added  to  the  solution  a  very  clear  solution  of  al- 
bumenates.  One  gram  of  the  dry  albumenates  was  sufficient 
for  a  hundred  grams  of  the  tartrate. 

Thereby  it  frequently  happened  that  the  fluid  was  at  once 
set  into  fermentation.  I  said  frequently  spontaneous  fermenta- 
tion is  obtained,  but  I  may  add  that  this  is  always  the  case  if  a 
small  amount  of  the  fluid,  in  which  fermentation  has  taken 
place,  be  added. 

31 


MEMOIRS     ON 

Thus  far  the  phenomena  are  not  unusual.  It  is  a  fermentable 
tartrate,  a  fact  that  is  well  known. 

But  if  we  place  ammonium  racemate  under  the  same  con- 
ditions fermentation  also  sets  in.  The  same  yeast  is  used  and 
everything  appears  as  though  the  process  were  exactly  similar  to 
that  with  the  dextro-tartaric  acid.  If,  however,  we  follow  the 
progress  of  the  operation  with  the  help  of  the  polariscope,  we 
recognize  very  soon  a  fundamental  difference  between  the  two 
experiments.  The  fluid,  at  first  inactive,  assumes  marked  Isevo- 
rotatory  power  which  increases  little  by  little  until  a  maximum 
is  reached.  Then  the  fermentation  is  interrupted.  There  is 
found  now  not  a  trace  of  the  dextro-tartaric  acid  in  the  fluid, 
which  when  evaporated  and  mixed  with  an  equal  volume  of  al- 
cohol gives  beautiful  crystals  of  ammonium  laevo-tartrate. 

Two  different  things  are  to  be  observed  in  these  phenomena: 
In  every  case  by  fermentation  a  substance  undergoes  chemical 
change,  and  at  the  same  time  an  organism  is  developed  which 
belongs  to  the  fungi.  On  the  other  hand  it  appears  that  the 
yeast  which  set  the  dextro-salt  into  fermentation  is  without 
action  upon  the  laevo-salt,  in  spite  of  the  absolute  identity  of 
the  physical  and  chemical  properties  of  both,  so  long  as  they 
are  subjected  to  nonasymmetric  influences.  Here  we  have  an 
organic  material,  possessing  molecular  asymmetry,  acting  upon 
a  physiological  process  in  such  a  way  as  to  show  that  it  has  pro- 
duced a  change  in  the  chemical  affinity.  There  cannot  be  the 
slightest  doubt  that  the  only  and  exclusive  cause  of  this  differ- 
ence in  the  fermentation  of  the  two  tartaric  acids  is  caused  by  the 
opposite  molecular  arrangement  of  the  laevo-tartaric  acid. 

In  this  way  the  idea  of  the  influence  of  the  molecular  asym- 
metry of  natural  organic  products  is  introduced  into  physiolog- 
ical studies,  this  important  characteristic  is  perhaps  the  only 
distinct  line  of  demarkation  which  we  can  draw  to-day  between 
dead  and  living  nature. 

XIII. 

THIS  is,  Gentlemen,  a  recapitulation  of  the  work  which  I  was 
commissioned  to  bring  before  you. 

You  will  in  the  course  of  these  lectures  understand  why  I 
have  given  them  the  title  "Upon  the  Asymmetry  of  Naturally 

32 


STEKEO-CHEMISTBY. 

Occurring  Organic  Compounds."  I  have  in  fact  set  up  a 
theory  of  molecular  asymmetry,  one  of  the  most  important  and 
wholly  surprising  chapters  of  the  science,  which  opens  up  a  new, 
distant,  but  definite  horizon  for  physiology. 

I  pronounce  this  judgment  upon  the  results  of  my  own  in- 
vestigations, without  allowing  the  discoverer's  feeling  of  self 
satisfaction  to  influence  this  expression  of  my  thoughts. 
Heaven  forbid  that  this  rostrum  ever  be  made  use  of  to  advance 
personal  ambitions.  These  lectures  are  intended  to  furnish 
glimpses  at  the  history  of  chemistry,  and  we,  the  lecturers, 
must  therefore  be  permeated  with  the  spirit  of  their  dignity  and 
with  impartiality,  as  only  these  can  infuse  true  life  into  science. 

BIOGRAPHICAL   SKETCH.1 

Louis  PASTEUR  was  born  at  Dole,  December  27,  1822,  and 
died  near  Paris  September  28,  1895.  In  1843  he  entered  the 
Ecole  Normale  at  Paris  and  attended  the  chemical  lectures  of 
Balard  and  Dumas. 

In  1848  he  was  called  as  physicist  to  the  Lyceum  of  Dijon, 
and  a  few  months  later  he  accepted  the  chair  of  chemistry  in 
the  University  of  Strassburg.  In  1854  he  became  Dean  of  the 
Faculty  ^at  Lille,  and  in  1857  he  returned  to  Paris  as  Director 
of  the  Ecole  Normale.  In  1862  he  became  a  member  of  the 
Institute  ;  in  1881  he  was  elected  to  the  French  Academy.  In 
1874  the  National  Assembly  voted  him  an  annual  pension  of 
20,000  francs.  In  1889  he  gave  up  all  of  his  public  offices  in 
order  to  devote  himself  to  the  management  of  the  famous 
Pasteur  Institute,  which  was  founded  by  public  subscriptions. 
Pasteur  was  preeminently  a  chemist,  but  all  departments  of 
natural  science  were  enriched  by  his  labors. 

Owing  to  his  work  with  the  chemistry  of  fermentation  and 
bacteriology  and  its  important  results  he  may  justly  be  called 
the  father  of  modern  scientific  medicine. 

1  See  Pasteur  Memorial  Lecture 
Jour..  Chem.  Soc.  71,683.  (1897). 


- 


33 


A  SUGGESTION  LOOKING  TO  THE  EXTEN- 
SION INTO  SPACE  OF  THE  STRUCTURAL 
FORMULAS  AT  PRESENT  USED  IN  CHEM- 
ISTRY. 

AND  A  NOTE  UPON  THE  RELATION  BE- 
TWEEN THE  OPTICAL  ACTIVITY  AND  THE 
CHEMICAL  CONSTITUTION  OF  ORGANIC 
COMPOUNDS. 

BY 

J.  H.  VAN'T  HOFF. 

Utrecht,  1874. 


35 


CONTENTS. 


Present  theory  not  in  accord  ivith  the  facts          .  37 

Brought  info  accord  ivith  the  facts  by  a  new  assumption       .     38 
Relation  between  the  asymmetric  carbon  atom  and  the  prop- 
erty of  optical  activity        .         .         .         .         .         .38 

Relation  between  the  asymmetric  carbon  atom  and  the  number 

of  isomers  .........     41 

Hypothesis  applied  to  ethylene  compounds     .         .        .         .42 

Hypothesis  applied  to  acetylene  compounds.  .         .         .44 

Biographical  Sketch .46 


36 


A  SUGGESTION  LOOKING  TO  THE  EXTEN- 
SION INTO  SPACE  OF  THE  STRUCTURAL 
FORMULAS  AT  PRESENT  USED  IN  CHEM- 
ISTRY. 

AND  A  NOTE  UPON  THE  RELATION  BE- 
TWEEN THE  OPTICAL  ACTIVITY  AND  THE 
CHEMICAL  CONSTITUTION  OF  ORGANIC 

COMPOUNDS. 

BY  J.  H.  VAN'T  HOFF. 

Utrecht,  1874. 

I  DESIRE  to  introduce  -some  remarks  which  may  lead  to  dis- 
cussion and  hope  to  avail  myself  of  the  discussion  to  give  to 
my  ideas  more  definiteness  and  breadth.  Since  the  starting 
point  for  the  following  communication  is  found  in  the  chem- 
istry of  the  carbon  compounds,  I  shall  for  the  present  do  noth- 
ing more  than  state  the  points  having  reference  to  it. 

It  appears  more  and  more  that  the  present  constitutional 
formulas  are  incapable  of  explaining  certain  cases  of  isomerism; 
the  reason  for  this  is  perhaps  the  fact  that  we  need  a  more 
definite  statement  about  the  actual  positions  of  the  atoms. 

If  we  suppose  that  the  atoms  lie  in  a  plane,  as  for  example 
with  isobutyl  alcohol  (Figure  I.)  where  the  four  affinities  are 
represented  by  four  lines  in  this  plane  occupying  two  directions 
perpendicular  to  one  another,  then  methane  (CEU)  (to  start 
with  the  simplest  case)  will  give  the  following  isomerie  modifi- 
cations (the  different  hydrogen  atoms  being  replaced  one  after 
the  other  by  univalent  groups  R'  R"  etc.): 

One  for         CH3R'  and  for  CHR'3 

Two  for        CH^R's  (Figures  II.  and  III.),  for 

CHaR'R",  and  for  CHR'oR" 

Three  for     CHR'R'R7"  and  for  CR'R"R'"R"" 

(Figures  IV.,  V.  and  VI.;) 

37 


MEMOIRS     ON 

numbers  that  are  clearly  greater  than  the  numbers  actually 
known  thus  far. 

The  theory  is  brought  into  accord  with  the  facts  if  we  con- 
sider the  affinities  of  the  carbon  atom  directed  toward  the 
corners  of  a  tetrahedron  of  which  the  carbon  atom  itself  oc- 
cupies the  center. 

The  number  of  isomers  is  then  reduced  and  will  be  as 
follows: 

One  for       CH3R',        CH2R'2,        CHaR'R",       CHR'3, 

and          CHR'2R"  but 

Two  for       CHR'R"R"/  or  more  general  for 

CR'R"R'"R"" 

If  one  imagines  himself  in  the  line  R'R'"  in  Figures  VII.  and 
VIII.  with  head  toward  R'  and  looking  toward  the  line  R"R'" 
then  R"  may  be  on  the  right  (Figure  VII.)  or  on  the  left  (Fig- 
ure VIII.)  of  the  observer  ;  in  other  words  :  When  the  four  af- 
finities of  the  carbon  atom  are  satisfied  by  four  univalent  groups 
differing  among  themselves,  two  and  not  more  than  tivo  different 
tetrahedrons  are  obtained,  one  of  which  is  the  reflected  image  of 
the  other,  they  cannot  be  superposed;  that  is,  we  have  here  to  deal 
with  two  structural  formulas  isomeric  in  space.  According  to 
this  hypothesis  the  combination  CR'R^R^R""  presents  a  con- 
dition not  presented  by  the  combinations  CR^R^R'",  CR'sR"  or 
CR'4  a  condition  not  expressed  by  the  ordinary  mode  of  repre- 
sentation. According  to  the  present  mode  there  would  be  be- 
tween CR'R^R'^R""  and  CR'iR"R'"  a  difference  quite  as  great  as 
between  CR's^'R'"  and  CR'3R",  or  between  CR'3R"and  CR'4. 

Submitting  the  first  result  of  this  hypothesis  to  the  control 
of  facts,  I  believe  that  it  has  been  thoroughly  established 
that  some  combinations  which  contain  a  carbon  atom  com- 
bined with  four  different  univalent  groups  (such  carbon  atoms 
will  henceforth  be  called  asymmetric  carbon  atoms)  present 
some  anomalies  in  relation  to  isomerism  and  other  character- 
istics which  are  not  indicated  by  the  constitutional  formulas 
thus  far  used. 

FIRST   PART.     I.     RELATION*    BETWEEN*    THE    ASYMMETRIC 

CARBON  AND  THE  PROPERTY  OF  OPTICAL  ACTIVITY. 

(a)  All  of  the  compounds  of  carbon  which  in  solution  rotate 
the  plane  of  polarized  light  possess  an  asymmetric  carbon  atom. 

38 


STEREO-CHEMISTRY. 

In  order  to  convince  oneself  of  the  justice  of  these  remarks  it 
is  necessary  to  run  through  the  following  list  of  optically  active 
compounds  in  the  formula  of  which  the  asymmetric  carbon 
is  indicated  by  C  : 

Ethylidene  lactic  acid,  CH3  C.H.  OH.  COOH. 

Aspartic  acid,  COOH  C.H.  NH2  (CH2  COOH). 

Asparagine,  COOH  C.H.  NH2.  (CH2  CONH2). 

Malic  acid,  COOH  C.  OH.  H.  (CH2  COOH). 

Glutaric  acid  [Itamalic  acid] 

CH2OH  C.H.  COOH.  (CH2  COOH). 

Tartaric  acid,  COOH  C.H  OH  C.H.  OH.  COOH. 

Dextrose,  Laevulose,  Galactose,  Maltose. 
Sorbin,  Eucalyn,  etc.,  CH2  OH.  C.H.  OH.  (C4H704) 

Mannite,  Quercite,  Finite  :  (C4H004).  C.  H.  OH.  CH2OH. 
Cane  sugar,  milk  sugar,  Melezitose,  Melitose,  Parasacchrose, 
and  Trehalose ;  Starch,  Inuline,  Glycogen,  Dextrine,  and 
Arabin  all  contain  the  asymmetric  carbon  atom  that  was  pres- 
ent in.  the  previous  compounds  inasmuch  as  they  are  com- 
pound ethers  of  the  previous  compounds. 

Camphor,  according  to  KEKULE  (Figure  XII.). 
Borneol,  according  to  the  same  (Figure  XIII.). 
Camphoric  acid,  according  to  the  same. 

COOH  CH  (C8H140.). 

Terpinolene  which  apparently  has  the  structure  shown  in 
Figure  XIV.  and  Menthol  which  perhaps  has  the  structure 
shown  in  Figure  XV. 

Concerning  the  active  alkaloids,  albumens,  etc.,  too  little  is 
as  yet  known  of  their  structure  to  permit  of  any  conclusion 
being  reached  in  regard  to  the  relation  between  their  structure 
and  the  rotatory  power. 

The  sole  definite  exception  to  this  rule  that  I  have  been  able 
to  find  is  the  active  propyl  alcohol  of  Chancell,  but,  according 
to  a  private  communication  of  Henniger,  this  relatively  small 
rotatory  power  is  due  to  the  presence  of  an  impurity. 

(b)  The  derivatives  of  optically  active  compounds  lose  their 
rotatory  power  when  the  asymmetry  of  all  of  the  carbon  atoms 
disappears  ;  in  the  contrary  case  they  do  not  usually  lose  this 
power. 

A  few  examples  will  be  sufficient  here  : 

39 


MEMOIRS     ON 

Inactive  malonic,  fumaric,  and  maleic  acids  from  the  active 
malic  acid  ;  inactive  succinic  and  tartronic  acids  from  the 
active  tartaric  acid  ;  inactive  cymene  from  active  camphor,  etc. 

In  the  contrary  case  there  are, 

Active  malic  acid  from  active  tartaric  acid  ; 

Active  tartaric  acid  from  active  lactose  ; 

Active  glucose  from  active  glucosides  ; 

Active  nitro  mannite  from  active  mannite  ; 

Active  camphoric  acid  and  Borneol  from  active  camphor  ; 

Active  salts  and  esters  from  active  acids,  etc. 

(c)  If  one  makes  a  list  of  compounds  which  contain  an 
asymmetric  carbon  atom  it  is  then  seen  that  in  many  cases  the 
converse  of  (a)  is  not  true,  that  is,  not  every  compound  with  such 
an  atom  has  ati  influence  upon  polarized  light. 

This  may  be  ascribed  to  three  causes  : 

1.  The  compounds  consist  of  an  inactive  mixture   of   two 
isomers  with  equal  but  opposite  optical  power,  which  owing  to 
their  close  agreement  in  all  other  properties  can   be  separated 
with  great  difficulty,  and  which  have  not  up  to  the  present  been 
separated. 

2.  The   study  of   the   rotatory  power  has  been  imperfect, 
either  on  account  of  the  slight  solubility  of  the  compounds  or 
on  account   of  the  slight  specific  rotatory  power  of  many  com- 
pounds, as  for  example,  in  the  case  of  mannite. 

3.  The  asymmetric  carbon  atom  may  not  in  itself  be  suffi- 
cient to  cause  optical  activity,  the  latter  may  not  depend  solely 
upon  the  mutual  diversity  of   the  groups  which    are  in  com- 
bination  with   the   carbon   atom,  but   may  also  be  dependent 
upon  their  character. 

However  the  case  may  be  the  facts  noted  indicate  a  probable 
relation  between  constitution  and  active  power  which  may  be 
made  use  of  in  the  following  cases  when  more  convincing 
arguments  fail  : 

1.  A  compound  which  rotates  the  plane  of  polarized  light 
probably  possesses  an  asymmetric  carbon  atom  ;  which  gives 
a  means  of  choosing  between  possible  structures  in  the  case  of 
compounds  where  the  structure  is  not  completely  determined. 

For  example,  active  amyl  alcohol  with  an  asymmetric  carbon 
atom  can  have  only  the  formula 

40 


STEREO-CHEMISTRY. 


CH.  CH2  OH 

i  2  * 

a  formula  which  has  also  been  suggested  by  Erlenmeyer  but 
upon  altogether  different  grounds. 

2.  A  compound  which  up  to  the  present  has  shown  no  physi- 
cal isomers  acting  upon  polarized  light  in  all  probability  con- 
tains no  asymmetric  carbon  atom  ;  this  fact  also  may  be  of 
service  in  choosing  between  possible  structural  formulas;  as, 
for  example,  citric  acid,  which  on  account  of  its  transformation 
into  aconitic  and  tricarballylic  acids  must  have  one  of  the  two 
formulas  : 

C.H.  OH.  COOH      CH2  COOH 

CH.  COOH     or    C.  OH.  COOH 

CH2  COOH          CH2  COOH 

its  inactivity  gives  preference  to  the  second  formula  ;  the  first, 
however,  contains  an  asymmetric  carbon  atom  for  which  reason 
I  hope  to  be  able  to  produce  the  acid  named  by  following  the 
method  of  Frankland  and  Duppa  from  oxalic  acid  and  iodo 
acetic  acid  esters  by  the  aid  of  zinc. 

3.  Finally  the  limits  of  the  rotatory  power  can  be  stated  with 
some  measure  of  probability,  that  is  to  say,  the  simplest  combi- 
nations which  will  show  active  power  can  be  indicated  ;  for  ex- 
ample, the  simplest  active  monatomic  alcohol  will  be: 

CH3.  C.  H.  OH.  CH2  CH3. 
The  simplest  active  monobasic  acid  : 

CH3.  C.  H.  COOH.  CH2  CH3. 
The  simplest  active  diatomic  alcohol: 

CH3CHOHCH2OH. 
The  simplest  active  saturated  hydrocarbon: 


J3  n.7 

The  simplest  active  aromatic  hydrocarbon  : 
^g3    CH.  C6H5  etc. 

At  the  same  time  it  is  probable  that  some  series  will  be  ex- 
cluded from  active  power,  as  for  example  : 
D 

41 


MEMOIRS     ON 

The  normal  hydrocarbons  CHa  (CH2)n  CH3 

The  normal  alcohols  CH3  (OH2)n  CHgOH 

The  normal  acids  OH3  (CH8)n  COOH     etc. 

It  is  more  noteworthy  that  inconsequence  of  the  assumptions 
made,  the  compound  OH  Br  01  I  can  probably  be  split  up  into 
two  isomers  which  will  act  upon  polarized  light. 

II.  RELATION  BETWEEN  THE  ASYMMETRIC  CARBON  ATOM  AND 
THE  NUMBER  OF  ISOMERS.  Since  perhaps  the  asymmetric  carbon 
atom  does  not  cause  all  compounds  in  which  it  is  present  to  be 
optically  active,  it  ought,  according  to  the  fundamental  hy- 
pothesis, to  cause  an  isomerism  which  will  show  itself 
in  one  way  or  another  ;  in  consequence  the  number  of  isomers 
foreseen  by  the  present  structural  formulas  will  be  doubled  by 
the  presence  of  a  single  asymmetric  carbon  atom,  and  is  further 
increased  by  the  presence  of  several. 

I  believe  that  there  are  compounds  which  show  this  apparent 
anomaly  which  Wislicenus  has  stamped  with  the  name  of  geo- 
metric isomerism,  at  the  same  time  he  pointed. to  the  unsatis- 
factory nature  of  the  present  ideas  without  in  any  way  proposing 
an  hypothesis  which  would  be  more  logical. 

Among  these  may  be  mentioned  the  ethylidene  lactic  acid 
which  lias  one  asymmetric  carbon  atom  ; 

Tartaric  acid,  dibro.rn-  and  isodibrom-succinic  acid,  citra-, 
ita-  and  mesa-brompyrotartaric  acid,  citra-,  ita-  and  mesa- 
malic  acid,  mannite  and  its  isomers,  dextrose  and  its  isomers, 
perhaps  also  terpinolene,  the  sugars,  etc.,  with  their  isomers,  in 
all  of  which  several  asymmetric  carbon  atoms  act  together  to 
increase  the  number  of  the  isomers. 

SECOND  PART.  Thus  far  we  have  considered  the  influence 
of  the  hypothesis  upon  compounds  in  which  the  carbon  atoms 
are  united  by  a  single  affinity  only,(leaving  out  some  aromatic 
bodies);  there  remains  now  to  be  considered  : 

The  influence  of  the  new  hypothesis  upon  compounds  containing 
doubly  linked  carbon  atoms.  Double  linking  is  represented  by 
two  tetrahedrons  with  one  edge  in  common  (Figure  IX.)  in  which 
A  and  B  represent  the  union  of  the  two  carbon  atoms,  and 
R'  R"  R'/x  R""  represent  the  univalent  groups  which  saturate  the 
remaining  free  affinities  of  the  carbon  atoms. 

If  R'  R"  R'"  R""  all  represent  the  same  group,  then  but  one 

42 


ST  E  RE  0  -  C  H  E  M  I  S  T  R  Y . 

form  is  conceivable,  and  the  same  is  true  if  R'  and  R"  or  R'" 
and  R""  are  identical,  but  if  R'  differs  from  R"  and  at  the  same 
time  R"  differs  from  R"",  which  does  not  preclude  R  and  R", 
R"  and  R""  from  being  equal,  then  two  figures  become  possible 
shown  in  Figures  IX.  and  X.,  which  differ  from  one  another  in 
regard  to  the  positions  of  R  and  R"  with  respect  to  R"  and  R"" 
the  dissimilar  ity  of  these  figures,  which  are  limited  to  two,  indi- 
cates a  case  of  isomerism  not  shown  by  the  ordinary  formulas. 

Turning  to  the  facts,  I  believe  that  I  have  met  with  such 
cases  among  organic  compounds. 

1.  Malei'c  and  fu marie  acids,  all  explanations  of  the  isomer- 
ism between  these  have  made  shipwreck,  (I  count  here  also  the 
assumption  of  a  bivalent  carbon  atom  since  this  can  exist  alone 
in  the  case  of  carbon  monoxide  and  the  carbylamines  for  evident 
reasons,  without  doubling  of  the  molecule)  ;  as  a  matter  of  fact 
these  acids  realize  the  conditions  outlined  above  :   Two  doubly 
linked  carbon  atoms  each  carrying  two  unlike  univalent  groups, 
Hand  CO  OH. 

2.  Brom  and   isobrom  maleic  acid,  the  explanation  of  the 
isomerism  here  is  entirely  the  same  as  before,  one  has  only  to 
replace  an  H  in  the  fumaric  and  maleic  acid  by  a  Br. 

3.  Citra-,  ita-  and  mesaconic  acids.     With  the  adoption  of 

CH3  CH  COOH  CH8  COOH 

for  pyrotartaric  acid  there  remains  for  the  acids  mentioned 
only  the  formulas 

CH2=C.  COOH.  CH2  COOH 

CH3  C  COOH=CH.  COOH 

and  if  the  latter  does  not  contain  two  isomers  (probably  ita- and 
citraconic  acids)  in  accordance  with  my  hypothesis,  no  plaus- 
ible explanation  can  be  given. 

4.  Solid  and  liquid  crotonic  acids.     The  constitution  of  the 
solid  crotonic  acid  according  to  Kekule  is  without  doubt 

CH3  CH=CH  COOH 

for  the  .liquid  crotonic  acid  there  remains  therefore  (thus  it  is 
held)  only  the  formula 

.      CH2=CH  CH2  COOH 
to  explain  their  lack  of  identity. 

But  if  we  take  into  consideration  the  following  facts  with 
regard  to  this  acid  : 

43 


MEMOIRS     OJST 

(a)  Fused  with  KOH   it  gives,  according  to  M.  Hemilian, 
acetic  acid  only. 

(b)  Oxidizing    agents,   according    to    the    same    authority, 
convert  it  into  acetic  and  oxalic  acids,  and  indirectly  from  the 
oxalic  acid  into  carbonic  acid. 

(c)  At  170° — 180°,  also  according  to  Hemilian,  it  goes  over  into 
the  solid  crotonic  acid.     Thus  there  is  nothing  in  favor  of  the 
fqrmula  CH^CH  CH2  COOH  and  everything  in  favor  of  the 
isomer  CH3  OH=CH   COOH,  exactly  like  fumaric  and  nialeic 
acids.     The  formula  CH3  CH=CH  COOH  really  satisfies  the 
conditions  exacted  by  my  hypothesis  for  the  possibility  of  two 
isomers  :  two  doubly  linked  carbon  atoms,  the  free  affinities  of 
each  of  which  are  saturated  by  two  unlike  univalent  groups,  in 
this  case  H  and  CH3,  H  and  COOH. 

Geuther's  chlorcrotonic  acid  and  chlorisocrotonic  acid,  the 
isomerism  of  which  has  hitherto  been  expressed  by  the  for- 
mulas 

CH2=  CC1  CH2  COOH 
and  CH3  001=  CHCOOH, 

according  to  Froelich  give  with  nascent  hydrogen  the   acids 
treated  of  under  (4)  whence  the  constitution  of  both  becomes 

CH8  CC1  =  CHCOOH 
and  this  case  of  isomerism  strengthens  my  hypothesis. 

THIRD  PART.  There  remain  now  to  be  treated  carbon 
atoms  which  are  united  by  a  triple  union  as  in  acetylene  ;  this 
combination  is  represented  by  two  tetrahedrons  with  three 
summits  in  common  or  with  one  of  their  faces  in  common 
(Figure  XL).  ACB  is  the  triple  union,  R'  and  R"  are  the  univ- 
alent groups  which  saturate  the  two  remaining  affinities  of 
the  carbon  atoms.  The  new  hypothesis  does  not  in  this  case 
lead  to  any  discordance  with  the  views  previously  held. 

In  closing  I  wish  to  remark  that 

1.  The  new  hypothesis  leaves  nothing  unexplained  that  is 
clearly  set  forth  by  the  previous  conceptions. 

2.  Certain  properties  and  isomers  not  explained  by  the  usual 
theories  receive  some  light  from  this  point  of  view. 

3.  Finally  my  remarks  about  active  compounds  in  solution, 
that  is  active  molecules,  are  related  to  the  views  of  Rammels- 
berg  upon  active  crystals. 

44 


STEREO-CHEMISTRY. 


Extending  the  observations  of  Herschell  and  Pasteur, 
Ramm els  berg  maintains  that  the  property  of  acting  upon  the 
plane  of  polarization  in  the  solid  state  (that  is  the  active  con- 
dition of  crystals  with  inactive  molecules  as  well  as  the  inactive 
condition  of  crystals  with  active  molecules)  coincides  with  the 
appearance  of  two  crystal  forms,  one  of  which  is  the  reflected 
image  of  the  other. 

It  is  evident  that  we  have  here  to  deal  with  an  arrangement 
of  the  molecules  in  the  active  crystal  altogether  similar  to  the 
arrangement  of  the  groups  of  atoms  in  the  active  molecule 
according  to  my  hypothesis  ;  an  arrangement  in  which  neither 
the  crystal  mentioned  by  Rammelsberg  nor  the  active  molecules, 
represented  in  a  general  way  by  Figures  VII.  and  VIII.  have  a 
plane  of  symmetry. 

H        I.  II.        R'  III.        R' 

H— C— H  H— C— H  H— C— R' 

H  R'  H 


H— C C— 0  H 


IV.     R 


V. 


H— C— H 


VI. 


•""—  C— R"    R""— C— R'"     R"— C— R' 
R'"  R"  R"" 


VIII. 


45 


MEMOIRS    ON    STEREO-CHEMISTRY. 
XII.  XIII.  XIV.  XV. 


J.  H.  YAN'T  HOFF. 

Utrecht,  Sept.  5,  '74. 

BIOGRAPHICAL  SKETCH. 

JACOBUS  HENRICUS  YAN'T  HOFF  was  born  in  Rotterdam 
August  30,  1852.  In  1869  he  entered  the  Polytechnic  In- 
stitute in  Delft,  and  completed  their  three  year  course  in  two 
years ;  he  then  went  to  the  University  of  Leyden  ;  in  1872  he 
was  at  Bonn  with  Kekule,  and  in  1873  he  was  with  Wiirtz  in 
Paris,  and  with  Mulder  in  Utrecht  in  1874  where  he  received 
his  degree  of  Ph.D.  He  was  made  Docent  in  the  veterinary 
college  in  Utrecht  in  1876.  In  1877  he  went  as  lecturer  in 
chemistry  to  Amsterdam  and  was  made  professor  in  the  follow- 
ing year.  He  remained  in  this  position  until  1894  when  he 
accepted  a  call  to  the  University  of  Berlin,  where  he  is  at 
present. 

While  the  number  of  van't  HofFs  contributions  to  chemistry 
is  not  so  very  large  an  extraordinary  large  proportion  of  them 
have  been  of  an  epoch  making  character. 

Important  among  these  are  his  Theory  of  the  Asymmetric 
Carbon  Atom.  His  studies  of  osmotic  pressure  and  his  point- 
ing out  of  the  analogy  between  gases  and  dilute  solutions;  and 
his  paper  on  Solid  Solutions  and  the  Determination  of  the 
Molecular  Weights  of  Solids,  have  opened  up  new  fields  of 
work. 

For  a  complete  account  of  van't  Hoff's  Life  and  Work  see 
the  "Jubelband"  Zeit.  fur.  Physikalische  Chemie,  Volume 
31. 

46       • 


ON  THE  RELATIONS  WHICH  EXIST  BE 

TWEEN  THE  ATOMIC  FORMULAS 

OF  ORGANIC  COMPOUNDS  AND 

THE  ROTATORY  POWER  OF 

THEIR  SOLUTIONS. 

BY 

J.  A.  LEBEL. 

Bulletin  de  la  Societe  Chimique  de  Paris,  24,  337. 
November,  1874. 


47 


CONTENTS. 


Development  of  the  fundamental  idea,  .         .  49 

Its  application  to  saturated  bodies,  .<        .  .         .         .51 

Its  application  to  unsaturated  bodies,  .  .  55 

Its  application  to  aromatic  bodies,  .        .  .  57 
Synthetical  substances  with  an  asymmetric  carbon  atom  will 

be  inactive,          .    .  ,-.'      .     .  ^ .  '.  .  ,        .        .58 

Biographical  Sketch       .       :.        .  60 


48 


ON    THE  RELATIONS    WHICH  EXIST  BE- 
TWEEN THE  ATOMIC  FORMULAS  OF 
ORGANIC   COMPOUNDS   AND   THE 
ROTATORY  POWER  OF  THEIR 
SOLUTIONS. 

BY  J.  A.  LEBEL. 

UP  to  the  present  time  we  do  not  possess  any  certain  rule 
which  enables  us  to  foresee  whether  or  not  the  solution  of  a 
substance  has  rotatory  power.  We  know  only  that  the  deriva- 
tives of  an  active  substance  are  in  general  also  active  ;  neverthe- 
less we  often  see  the  rotatory  power  suddenly  disappear  in  the 
most  immediate  derivatives,  while  in  other  cases  it  persists  in 
very  remote  derivatives.  By  considerations,  purely  geometri- 
cal, I  have  been  able  to  formulate  a  rule  of  a  quite  general 
character. 

Before  giving  the  reasoning  which  has  led  me  to  this  law  I 
shall  give  the  facts  upon  which  it  rests,  and  then  shall  conclude 
with  a  discussion  of  the  confirmation  of  the  law  offered  by  the 
present  state  of  our  chemical  knowledge. 

The  labors  of  Pasteur  and  others  have  completely  established 
the  correlation  which  exists  between  molecular  asymmetry  and 
rotatory  power.  If  the  asymmetry  exists  only  in  the  crystal- 
line molecule,  the  crystal  alone  will  be  active ;  if,  on  the  con- 
trary, it  belongs  to  the  chemical  molecule  the  solution  will 
show  rotatory  power,  and  often  the  crystal  also  if  the  structure 
of  the  crystal  allows  us  to  perceive  it,  as  in  the  case  of  the  sul- 
phate of  strychnine  and  the  alum  of  amylamine. 

There  are,  moreover,  mathematical  demonstrations  of  the 
necessary  existence  of  this  correlation,  which  we  may  consider 
a  perfectly  ascertained  fact. 

In  the  reasoning  which  follows,  we  shall  ignore  the  asym- 
metries which  might  arise  from  the  arrangement  in  space  pos- 

49 


MEMOIRS     ON 

sessed  by  the  atoms  and  univalent  radicals  ;  but  shall  consider 
them  as  spheres  or  material  points,  which  will  be  equal  if  the 
atoms  or  radicals  are  equal,  and  different  if  they  are  different. 
This  restriction  is  justified  by  the  fact,  that,  up  to  the  present 
time,  it  has  been  possible  to  account  for  all  the  cases  of  iso- 
merism  observed  without  recourse  to  such  arrangement,  and  the 
discussion  at  the  end  of  the  paper  will  show  that  the  appearance 
of  the  rotatory  power  can  be  equally  well  foreseen  without  the 
aid  of  the  hypothesis  of  which  we  have  just  spoken. 

First  general  principle. — Let  us  consider  a  molecule  of  a 
chemical  compound  having  the  formula  M  A4  ;  M  being  a 
simple  or  complex  radical  combined  with  four  univalent  atoms 
A,  capable  of  being  replaced  by  substitution.  Let  us  replace 
three  of  them  by  simple  or  complex  univalent  radicals  differing 
from  one  another  and  from  M;  the  body  obtained  will  be  asym- 
metric. 

Indeed,  the  group  of  radicals  R,  R',  R",  A  when  considered 
as  material  points  differing  among  themselves  form  a  structure 
which  is  enantimorphous  with  its  reflected  image,  and  the  resi- 
due, M,  cannot  re-establish  the  symmetry.  In  general  then  it 
may  be  stated  that  if  a  body  is  derived  from  the  original  type 
M  A4  by  the  substitution  of  three  different  atoms  or  radicals  for 
A,  its  molecules  will  be  asymmetric,  and  it  will  have  rotatory 
power. 

But  there  are  two  exceptional  cases,  distinct  in  character. 

(1)  If  the  molecular  type  has  a  plane  of  symmetry  containing 
the  four  atoms  A,  the  substitution  of  these  by  radicals  (which  we 
must  consider  as  not  capable  of  changing  their  position)  can  in 
no  way  alter  the  symmetry  with   respect  to  this  plane,  and  in 
such  cases  the  whole  series  of  substitution  products  will  be  in- 
active. 

(2)  The  last   radical  substituted  for   A  may  be  composed  of 
the  same  atoms  that  compose  all  of  the  rest  of  the  group  into 
which  it  enters,  and  these  two  equal  groups  may  have  a  neutra- 
lizing effect  upon   polarized  light,   or  they  may  increase   the 
activity  ;  when  the  former  is  the  case  the  body  will  be  inactive. 
Now  this  arrangement  may  present  itself  in  a  derivative  of  an 
active  asymmetric  body  where  there  is  but  slight  difference  in 
constitution,  and  later  we  shall  see  a  remarkable  instance  of  this. 

50 


STEREO-CHEMISTRY. 

Second  general  principle. — If  in  our  fundamental  type  we  sub- 
stitute but  two  radicals  R,  R',  it  is  possible  to  have  symmetry 
or  asymmetry  according  to  the  constitution  of  the  original  type 
M  A4.  If  this  molecule  originally  had  a  plane  of  symmetry 
passing  through  the  two  atoms  A  which  have  been  replaced  by 
R  and  R',  this  plane  will  remain  a  plane  of  symmetry  after  the 
substitution  ;  the  body  obtained  will  then  be  inactive.  Our 
knowledge  of  the  constitution  of  certain  simple  types  will 
enable  us  to  assert,  that  certain  bodies  derived  from  them  by 
two  substitutions  will  be  inactive. 

Again,  if  it  happens  not  only  that  a  single  substitution  fur- 
nishes but  one  derivative,  but  also  that  two  and  even  three 
substitutions  give  only  one  and  the  same  chemical  isomer,  we 
are  obliged  to  admit  that  the  four  atoms  A  occupy  the  angles 
of  a  regular  tetrahedron,  whose  planes  of  symmetry  are.identi- 
cal  with  those  of  the  whole  molecule  M  A4  ;  in  this  case  also  no 
bisubstitution  product  can  have  rotatory  power. 

Application  to  the  saturated  bodies  of  the  fatty  series. — 
All  of  the  saturated  bodies  of  the  fatty  series  are  derived  from 
marsh  gas,  CH4,  by  the  replacement  of  the  hydrogen  by  differ- 
ent radicals.  Provided  that  the  four  atoms  of  hydrogen  are 
not  all  in  the  same  plane,  a  supposition  upon  which  the  very 
existence  of  active  trisubstitution  products  is  based,  we  are 
able  to  apply  -the  first  general  principle  and  assert  that  the 
substitution  of  three  different  radicals  will  furnish  active  bodies. 
Thus,  if  in  the  constitutional  formula  of  a  substance,  we  find  a 
carbon  atom  combined  with  three  univalent  radicals  different 
from  each  other  and  from  hydrogen,  we  ought  to  find  it  an 
active  body. 

Further  as  marsh  gas  never  furnishes  more  than  one  deriva- 
tive by  two  and  three  substitutions  we  can  apply  to  the  deriva- 
tives formed  by  two  substitutions  the  second  general  principle 
and  assert  that  such  derivatives  are  never  active  ;  thus  if  in  a 
constitutional  formula  we  see  a  carbon  atom  combined  with 
two  atoms  of  hydrogen  or  with  two  identical  radicals,  such  a 
body  ought  not  to  show  rotatory  power. 

Let  us  now  pass  in  review  the  active  bodies  of  the  fatty 
series. 

Lactic  group. — Lactic  acid  is  derived  from  marsh  gas  by  the 
substitution  of  a  part  of  the  hydrogen  by  the  three  groups  HO, 

51 


M  E  M  OIKS     ON 

COOH,  and  CHs  differing  from  one  another,  for  this  acid  has 
the  formula 

H 

CH3-C-COOH 

HO 

the  central  carbon  atom  is  in  this  case  one   to  which   the  first 
general  principle  can  be  applied,  this  body  ought  to  be  active. 

Mr.  Wislicenus  has,  in  fact,  recently  announced  that  he  has 
found  an  active  acid  in  meat.  This  acid  does  not  have,  as  has 
been  believed,  the  constitution  of  the  ethylene  lactic  acid  ;  but 
it  is  according  to  this  author  a  physical  isomer  of  the  ordinary 
lactic  acid.  Indeed  one  cannot  see  that  the  ethylene  lactic 
acid  can  be  active,  for  the  carbons  of  the  radical  are  each  one 
combined  with  two  atoms  of  hydrogen,  as  the  formula  shows  : 
Ca2  OH  CH2  COOH.  We  see  also  that  the  propylene  glycol 
and  the  iodopropionic  acid  which  are  derived  from  the  active 
lactic  acid  will  preserve  the  rotatory  power,  but  it  will  not  be 
the  same  with  the  glycerine  which  one  can  derive  from  it,  for 
in  that  case  the  central  carbon  atom  is  united  with  the  two 
equal  radicals  CH2  OH. 

Malic  group. — Malic  acid  presents  a  character  altogether 
analogous  ;  its  formula  places  its  rotatory  character  in 
evidence. 

H 

COOH-CH2-C-COOH 

0  II 

It  is  the  same  for  asparagine  which  is  derived  from  malic 
acid  by  the  substitution  of  NH2  for  two  of  the  hydroxyls  of 
the  latter.  Indeed,  aspartic  acid  which  is  still  active  contains 
a  single  NH2  in  the  place  of  the  central  hydroxyl. 

If,  on  the  other  hand,  we  replace  this  hydroxyl  by  H,  we 
form  succinic  acid,  which,  like  the  ethylene  lactic  acid,  is 
inactive. 

Tar taric  group. — Tartaric  acid  has  the  formula  : 


*] 

COOH-0    I 
II  J 


C-COOH 
HO 


52 


S  T  E  R  E  0  -  0  H  E  M  I  8  T  JJ  Y  . 

It  may  be  considered  as  derived  from  marsh  gas  by  the  sub- 
stitution of  the  hydrogen  by  the  three  univalent  radicals  : 

H 
HO     COOH,    and        COOH-6-,  consequently  it  should 

HO 

manifest  rotatory  power,  and  indeed  this  is  just  what  it 
does.  Moreover,  an  examination  of  the  formula  shows  that 
the  last  of  the  substituted  groups  is  identical  with  the  group- 
ing of  the  entire  remainder  of  the  compound,  we  have  to  deal 
therefore,  with  the  second  class  of  exceptions  to  the  first  gen- 
eral principle.  Two  arrangements  of  an  inverse  symmetry 
being  possible  in  this  grouping,  if  the  two  groups  combined 
with  one  another  are  identical  and  superposable  their  effect 
upon  the  polarized  light  will  be  added,  this  is  what  takes  place 
with  the  active  acid  ;  if,  on  the  contrary,  the  combined  groups 
have  an  inverse  symmetry  they  will  exactly  neutralize  one 
another  and  we  will  have  the  inactive  tartaric  acid.  This 
reasoning  applies  to  the  derivatives  of  tartaric  acid,  and  in 
particular  to  erythrite,  as  yet,  however,  only  the  inactive 
erythrite  is  known,  it  may  be  that  the  active  erythrite  is  not 
a  stable  body. 

Amylic  group.  —  The  active  amyl  alcohol  has  the  formula  : 

OgHs 

CH3-(5-CH2OH 
ft 

Its  rotatory  power  can  be  foreseen  from  its  formula  in  the 
same  manner  as  that  of  the  preceding  bodies.  This  substance 
gives  a  very  numerous  series  of  derivatives  which  show  the 
rotatory  power,  and  this  characteristic  is  shown  in  their  for- 
mulas. As  examples  we  may  cite  all  of  the  ethers  of  amyl 
alcohol,  and  the  acid  derived  from  it,  valeric  acid  : 


the  valerates,  the  valeric  ethers,  amylamine  and  nearly  all  of 
the  hydrocarbons  containing  the  radical  amyl,  such  as  the  ethyl- 
amyl,  diamyl,  etc.  It  is  no  longer  the  same  with  the  hydride  of 
amyl  : 

53 


MEMOIKS     ON 

C8H6 

CH3-C-CH3 
H 

We  see  in  fact,  that  it  contains  two  methyl  groups  united  to 
the  same  carbon  ;  this  body  is,  indeed,  inactive. 

This  fact  proves  that  the  presence  of  the  amyl  radical  does 
not  necessitate  the  rotatory  power  in  all  of  its  combinations. 

Mr.  Wiirtz  has  shown  that  the  caproic  acid  derived  from  the 
active  amyl  cyanide  has  rotatory  power.  If  we  compare  this  acid 
with  the  active  valeric  acid,  the  formula  of  which  has  already 
been  given,  we  see  that  it  is  derived  from  am}d  alcohol  just  as 
valeric  acid  is  derived  from  the  secondary  butyl  alcohol  of  Mr. 
de  Ltiynes.  We  are  able  to  conclude  empirically,  therefore, 
that  this  last  alcohol  is  active  ;  we  arrive  directly  at  the  same 
result  by  applying  the  first  general  principle  to  the  formula  of 
this  secondary  alcohol  which  is  : 

CaH5 

CHs-C-OH 
H> 

The  author  occupies  himself  with  the  preparation  of  many 
derivatives  of  this  group  in  order  to  verify  their  action  upon 
polarized  light. 

The  sugar  group. — The  general  constitution  of  the  sugars  is 
known,  but  their  exact  formulas  have  not  yet  been  given  ;  we 
must  therefore  confine  ourselves  to  the  prediction  of  facts  based 
upon  the  general  formulas. 

In  all  the  sugars  we  usually  find  a  carbon  united  with  hydro- 
gen, with  hydroxyl,  and  with  two  complex  radicals ;  if  the 
radicals  are  different,  as  very  generally  happens,  the  sugar  in 
question  should  be  active.  It  has  been  observed,  in  fact,  that 
most  sugars  possess  rotatory  power. 

The  sugars  are  naturally  divided  into  hexatomic  alcohols, 
such  as  mannite,  and  into  glucoses.  Let  us  consider  the  nor- 
mal hexatomic  alcohol 

II 
CH2  OH  CHOH  CHOH  CHOH-C-CH2  OH ;  we  see  that  each 

OH 

of  the  four  central  carbon  atoms  possesses  the  characteristics  of 

54 


S  T  E  R  E  0  -  C  H  E  M  1  S  T  K  Y  . 

rotatory  power,  although  in  the  formula  above  given  this  is 
indicated  with  the  fifth  carbon  atom  only. 

If  the  only  cause  of  the  asymmetry  of  this  molecule  were  the 
mode  of  arrangement  of  the  radicals  surrounding  a  single  one 
of  these  four  carbons,  we  should  have  a  laevo  and  a  dextro  body 
only  ;  but  as  four  similar  groupings  exist  there  will  be  as  many 
isomers  as  one  can  imagine  combinations  among  these  group- 
ings. This  is  a  repetition  of  the  facts  which  we  have  already 
met  with  in  tartaric  acid,  but  in  this  case  all  of  the  isomers  are 
not  yet  know. 

It  should  be  observed  that  the  groups  immediately  surround- 
ing the  central  carbon  atoms  in  the  above  formula  are  the  same, 
that  is  to  say,  the  radicals  with  which  they  are  combined  are 
either  identical  such  as  H,  and  OH,  or  they  differ  but  slightly 
in  their  most  remote  parts  ;  if  then  the  similar  groupings -give 
rise  to  rotatory  power  in  contrary  directions,  we  see  that  they 
will  compensate  one  another,  or  nearly  so  ;  so  that  the  rotatory 
power  of  the  whole  molecule  will  be  nothing,  or  very  slight. 
This  is  perhaps  the  explanation  of  the  fact  that  the  rotatory 
power  of  mannite,  dulcite,  and  their  hexa-nitrate,  and  hexa- 
acetate  is  so  much  reduced. 

This  supposition  which,  however,  is  independent  of  the 
general  theory,  finds  confirmation  in  the  fact  that  the  glucoses 
have  a  much  greater  rotatory  power  ;  now  the  glucoses  have 
an  aldehydic  or  a  ketonic  function  ;  let  us  consider  the  normal 
aldehydic  glucose  : 

H 
CH8  OH  CHOH  CHOH  CHOH-C-COH  : 

Oli 

we  see  that  the  asymmetric  grouping  which  surrounds  the  fifth 
carbon  is  completely  different  from  all  of  the  others,  and  there 
is  no  longer  any  reason  that  its  effect  upon  polarized  light 
should  be  compensated  by  that  of  the  neighboring  groups.  We 
understand  now  how  the  hydrogenation  of  very  active  glucoses 
may  give  hexatomic  alcohols  almost  destitute  of  rotatory 
power. 

Fatty  bodies  with  tivofree  valences. — We  have  not  yet  consid- 
ered active  un saturated  bodies,  for  we  do  not  include  in  this  class 
bodies  which  are  formed  by  the  substitution  of  an  active  radical 

55 


MEMOIRS     ON 

in  an  unsaturated  inactive  body,  such  as,  for  example,  allyl 
valerate. 

We  have  but  to  examine  the  case  where  the  double  linking  of 
the  unsaturated  body  is  caused  by  the  disappearance  of  some  of 
the  radicals,  the  asymmetrical  grouping  of  which  caused  the 
rotatory  power  in  the  corresponding  saturated  body. 

Since  all  the  substances  with  two  free  valences  are  derived 
from  ethylene,  it  is  to  this  latter  we  must,  if  possible,  apply  the 
general  principles  which  we  have  previously  employed.  We 
shall  put  aside  the  case  where  the  four  atoms  of  hydrogen  do 
not  have  fixed  positions  with  regard  to  one  another,  for  it  is 
clear  that  in  such  a  case  their  substitution  would  not  furnish 
asymmetric  bodies.  If,  on  the  contrary,  these  relative  positions 
are  fixed,  we  can  apply  to  ethylene  the  same  reasoning  that 
was  applied  to  marsh  gas. 

If  the  four  atoms  of  hydrogen  lie  in  the  same  plane,  which  is 
a  possible  case  of  equilibrium,  there  will  be  no  active  trisubsti- 
tution  derivatives  ;  however,  we  do  not  know  examples  of  well 
studied  bodies  derived  from  ethylene  by  three  different  sub- 
stitutions, and  we  are  therefore  unable  to  solve  this  question  at 
present. 

As  to  the  second  general  principle,  it  is  not-  applicable  to 
ethylene,  for  the  formula  CH2=CH2  shows  that  by  two  different 
substitutions  chemically  different  isomers  are  obtained.  This 
is  not  opposed  to  the  atoms  being  in  the  same  plane,  in  which 
case  the  derivatives  formed  by  two  substitutions  will  be  inac- 
tive. In  any  other  case,  to  explain  the  isomerism  of  the 
ethylene  derivatives,  we  must  suppose  the  hydrogen  atoms  to 
be  at  the  angles  of  a  hemihedral  quadratic  pyramid  superposa- 
p 

ble  upon  its  image-^-,  and  we  should  obtain  by  two  substitu- 
<i 

tions  two  isomers,  one  of  which  would  be  symmetrical  and  the 
other  unsymmetrical.  These  isomers  will  both  be  symmetrical 
if  the  two  substituted  radicals  are  the  same,  as  happens  in  the 
case  of  malei'c  and  fumaric  acids.  Hence  it  is  sufficient  for 
the  optical  study  of  two  bisubstitution  derivatives,  such  as  the 

amylene  of  active  amyl  alcohol  CH2=C  ™ii3,    and  its    isomer 

CH3CH==CH-C2H5,  to  decide  whether  the  four  hydrogen  atoms 
are  in  the  same  plane  or  not. 

56 


S  T  E  KE  0  -  0  H  E  MIS  T  ft  Y . 

Aromatic  series. — All  chemists  agree  in  the  opinion  that  the 
atoms  of  hydrogen  in  benzene  occupy  fixed  positions  ;  we  can 
no  longer  consider,  as  we  did  in  the  case  of  the  saturated 
bodies,  a  part  of  the  benzene  molecule  as  a  single  material 
point  ;  but  we  make  use  of  this  restrictive  hypothesis  again  in 
the  case  of  the  radicals  or  groups  which  replace  hydrogen  in 
benzene.  The  geometrical  hypotheses  which  account  for  the 
isomerism  of  the  aromatic  series  have  already  been  discussed 
elsewhere  ;  they  assume  that  the  six  atoms  of  hydrogen  are 
situated  at  the  six  equal  angles  of  a  rhombohedron,  or  perhaps 
at  those  of  a  right  prism  (pyramid)  with  an  equilateral  trian- 
gular base..  Simple  geometrical  reasoning  shows  that  in  either 
case  by  two  different  substitutions  there  are  obtained  one  asym- 
metrical and  two  different  symmetrical  isomers  ;  the  existence 
of  an  active  cymene  which  has  been  described  confirms  these 
hypotheses,  which  we  will  not  discuss  farther* 

Without  ascribing  to  the  atoms  of  hydrogen  in  benzene  any 
particular  grouping,  we  can  apply  the  first  general  principle  to 
any  three  atoms  of  hydrogen  whatever,  provided  that  they  do 
not  occupy  a  plane  of  symmetry  of  the  whole  molecule.  Hence 
it  follows  that  we  shall  meet  witli  active  bodies  whenever  at 
least  three  atoms  of  hydrogen  are  replaced  by  different  radicals. 
We  find  this  assumption  realized  in  a  large  number  of  sub- 
stances in  the  camphor  series.  (See  for  their  formulas  the 
work  of  Mr.  Kekule,  Bull.  Soc.  chim.,  t.  XX.  1873,  p.  558.) 
Camphor,  for  example,  is  derived  from  benzene  by  the  substitu- 
tion of  hydrogen  by  the  following  groups  : 


CH3,  °">     H2,andH2; 


three  of  them  are  different  and  the  two  others  are  not  able  to 
re-establish  the  symmetry  ;  it  has  been  proved  in  fact  that  all  of 
these  bodies  are  active. 

The  case  is  not  the  same  for  the  spirit  of  turpentine  ;  we 
know  that  this  is  derived,  as  is  also  the  camphor  series,  from 
para-cymene,  in  which  the  radicals  methyl  and  propyl  occupy 
positions  1  and  4  of  Mr.  Kekule's  hexagon,  that  is  to  say  in  a 
plane  of  symmetry  of  benzene,  and  this  is  the  reason  why  cymene 
is  inactive. 
E 

57 


MEMOIRS     ON 


Now  the  spirit  of  turpentine  is  derived  from  cymene  by  the 
substitution  of  two  H2  groups  for  two  atoms  of  hydrogen  ;  if 
these  occupy  the  positions  2  and  6  or  3  and  5,  symmetrical  with 
regard  to  the  plane  of  symmetry  passing  through  1  and  4,  we 
shall  have  inactive  isomers  ;  on  the  other  hand  we  shall  have 
active  isomers  (terebenthene  and  camphene)  if  they  occupy 
positions  not  symmetrical  to  one  another,  as  2  and  5  or  2  and 
3  ;  we  might  apply  the  same  reasoning  to  the  other  isomers  of 
terebenthene,  (for  the  formulas  of  these  see  the  memoir  of  Mr. 
Oppenheim,  loc.  cit.  p.  560). 

It  is  possible  that  camphoric  acid,  the  constitution  of  which 
is  not  yet  fully  established,  comes  under  the  preceding  case. 

Quinnic  acid  which  is  equally  active  is  derived  from  benzene 
by  a  different  but  insufficiently  understood  mode  of  substitu- 
tion, we  shall  not,  therefore,  discuss  it  here. 

We  see  what  interest  is  attached  to  the  study  of  active  aro- 
matic compounds,  and  how  necessary  it  is  that  chemists  who  are 
dealing  with  bi-  and  trisubstitution  products  of  benzene  capa- 
ble of  being  active  should  attempt  the  separation  of  their  dex- 
tro-  and  laevorotatory  isomers.  We  shall  proceed  to  show  that 
bodies  obtained  by  synthesis  consist,  in  fact,  of  equal  proportions 
of  these  isomers. 

Theorem. — When  an  asymmetric  body  is  formed  in  a  reaction 
where  there  are  present  originally  only  symmetrical  bodies,  the 
two  isomers  of  inverse  symmetry  will  be  formed  in  equal  quan- 
tities. 

We  know  that  the  general  principle  of  the  calculation  of 
probabilities  consists  in  this  : 

When  any  phenomenon  whatever  can  take  place  in  two  ways 
only,  and  there  is  no  reason  why  it  should  take  place  in  one  of 
the  ways  in  preference  to  the  other,  if  the  phenomenon  has  taken 
place  m  times  in  one  manner  and  m'  times  in  the  other  manner  the 

ratio — 7  approaches  unity  as  the  sum  m+tri  approaches  infinity. 

58 


STEREO-CHEMISTRY. 

When  an  asymmetric  body  has  been  formed  by  substitution 
from  a  symmetric  one,  the  asymmetry  has  been  introduced  by 
one  of  the  substitutions  which  has  taken  place;  let  us  consider 
this  point  carefully.  The  radical  or  the  atom  the  substitution 
of  which  introduced  the  asymmetry  had  formerly  a  homologue 
which  was  symmetrical  to  it  by  its  connection  with  a  point  or 
a  plane  of  symmetry;  these  radicals  being  in  similar  dynamic 
and  geometrical  conditions,  if  m  and  m'  represent  the  number 

/YY) 

of  times  that  each  one  of  them  is  substituted/ — —ought  to  ap- 
proach unity  as  the  number  of  these  substitutions  grows  beyond 
a  measurable  limit. 

Now  if  the  substitution  of  one  of  these  similar  radicals  pro- 
duces a  dextro-body  then  the  other  will  produce  the  laevo-body, 
both  will  in  consequence  be  formed  in  equal  proportions. 

It  is  the  same  for  asymmetric  bodies  formed  by  addition;  in- 
deed the  body  which  destroys  the  symmetry  of  a  symmetrical 
molecule  by  adding  itself  to  it,  would  be  able  to  occupy  an 
identical  place  situated  on  the  other  side  of  the  point  or  plane 
of  symmetry;  the  preceding  reasoning  therefore  can  be  applied 
equally  well  to  this  case. 

This  is  not  necessarily  true  of  asymmetric  bodies  formed  in 
the  presence  of  other  active  bodies,  or  traversed  by  circularly 
polarized  light,  or,  in  short,  when  submitted  to  any  cause  what- 
ever which  favors  the  formation  of  one  of  the  asymmetric 
isomers.  Such  conditions  are  exceptional;  and  generally  in  the 
case  of  bodies  prepared  synthetically  those  which  are  active 
will  escape  the  observation  of  the  chemist  unless  he  endeavors 
to  separate  the  mixed  isomeric  products,  the  combined  action 
of  which  upon  polarized  light  is  neutral. 

We  have  a  striking  example  of  this  in  tartaric  acid,  for 
neither  the  dextro-  nor  the  laevo- tartaric  acid  has  ever  been  ob- 
tained directly  by  synthesis,  but  the  inactive  racemic  acid 
which  is  a  combination  of  equal  parts  of  the  dextro  and  laevo 
acids,  is  always  obtained. 


59 


MEMOIRS     ON     STEREO-CHEMISTRY. 


BIOGRAPHICAL  SKETCH. 

JULES  ACHILLE  LE  BEL  was  born  at  Pechelbroun,  Alsace, 
January  21,  1847. 

He  ^studied  chemistry  and  physics  at  the  Lycee  Charlemagne, 
the  Ecole  Polytechnique  and  under  Wurtz  in  the  Ecole  de 
Medicine. 

He  owned  large  interests  in  the  oil  deposits  of  Pechelbroun, 
which  he  sold  ;  and  since  then  he  has  lived  privately  in  Paris 
for  the  most  part,  devoting  himself  to  chemical  studies.  His 
first  investigations  were  in  connection  with  the  petroleum  of 
Pechelbroun,  He  soon  became  interested  in  optically  active 
compounds  and  was  early  led  to  his  theory  of  the  asymmetric 
carbon  atom.  Much  of  his  subsequent  work  has  been  devoted  to 
the  experimental  confirmation  of  this  theory,  and  to  the  exten- 
sion of  the  views  to  ammonia  compounds. 


60 


THE    SPACE     ARRANGEMENT      OF     THE 

ATOMS  IN  ORGANIC  MOLECULES  AND 

THE    RESULTING     GEOMETRICAL 

ISOMERISM  IN  UNSATURATED 

COMPOUNDS. 

Abh.  K.  Sachs.  Ges.  der  Wissenschaften,  14.  1.  Leipsig,  1887. 

BY 
JOHANNES  WISLICENUS. 


61 


CONTENTS. 

PAGE 

/.     Introduction      .                         .....  65 

//.     Necessary  extension  of  the  theory  concerning  the  space 
relation  of  the  elementary  atoms  in  organic  com- 
pounds .         .         .        •         ...         .        .  68 

Geometric  properties  of  a  carbon  atom  system     .        f  69 

Geometric  properties  of  a  double   system         .        .  69 
Cases  of  geometric  isomerism  between  doubly  linked 

double  systems               •        .         .        .        .        »  70 

Method  of  determining  positions        .        •       "  •        •  73 
Rotation  of  singly  linked  system  owing  to  variation 
in  affinities    .      _ .        .        .        .        . '       .        .  '    75 

Rotation  of  the  same  in  consequence  of  heat  impulses  76 
TJie  formation  of  asymmetric  carbon  systems  from 

unsaturated  compounds  and  the  reverse  processes  .  78 

Different  kinds   of  symmetrical  positions        .        .  80 
Corresponding  positions    .        .        .                 .         .82 

///.     Special  part :    Application  of  the  hypothesis  to  single 

groups  of  chemical  facts          .        .      ..  .-82 

A.    Relative  space  relations  of  the  atoms  in  double  systems  83 

1.  Tlie  unsaturated  hydrocarbons  and  their  substi- 
tution products       .        .        .        .        .        .        .  83 

2.  Fumaric  and  maleic  acids  and  their  derivatives    .  87 

3.  TJie  pyrocitronic  acids  and  their  derivatives  .         .  96 

4.  The  unsaturated  acids  of  the  acrylic  acid  group    .  98 

(a)  Crotonic  acid  and  methyl  acrylic  acid         .        •  99 

(b)  Methyl  crotonic  acid  and  its  homologues     .        •  103 

(c)  Cinnamic  acid,  and  its  isomers  and  homologues  105 

(d)  Coumarin,    coumarinic    acid     and    coumaric 

acid     ~.        .         .        .        .         .         .        .        .  107 

5.  Transformation   of  unsaturated  compounds  into 
geometric  isomers  by  the  action  of  heat  .         .         .111 

63 


CONTENTS. 

B.    The  space  relations  with   three  and  more  carbon  atom 

systems ........  113 

1.     The  simultaneous  elimination  of    metal  haloides 
and  carbonic  acid  from  the  salts  of  halogen  sub- 
stituted acids         .....  113 

•2.     The  formation  of  lactones  and  the  anhydrides  of 

dibasic  acids.      The  behavior  of  a  and  (3  oxy  acids  .     123 

IV.     Conclusions 129 

Biographical  Sketch  of  Wislicenus        .  132 


64 


THE     SPACE     ARRANGEMENT     OF     THE 

ATOMS  IN  ORGANIC  MOLECULES  AND 

THE  RESULTING  GEOMETRICAL 

ISOMERISM  IN  UNSATURATED 

COMPOUNDS. 

1.     Introduction. 

§1.  Since  the  publication  of  the  theory  of  the  asymmetric 
carbon  atom  (1874)  by  van't  Hoff  and  at  the  same  time  by  Le 
Bel,  the  unavoidable  necessity,  shortly  before  recognized,1  of 
referring  back  the  difference  between  structurally  identical 
molecules  to  a  deviation  in  the  arrangement  in  space  of  their 
elementary  atoms  united  in  the  same  order  of  succession,  has 
made  but  little  advance.  The  advance  consists  in  the  proof  of 
the  fact  that  optically  active  organic  compounds  of  known 
structure  always  contain  an  asymmetric  carbon  atom,  and  that 
the  giving  up  of  asymmetry  is  at  once  accompanied  with  a  loss 
of  optical  activity;2  and  that  bodies  which  are  optically  inactive 
in  spite  of  the  presence  of  an  asymmetric  carbon  atom,  are 
mixtures  or  compounds  of  oppositely  active  modifications8. 

In  recent  times  the  theory  has  received  farther  extension  by 
BaeyerV  clear  explanation  of  the  surprising  relation  between 
the  thermal  values,  of  the  single,  double,  and  triple  union  of 
two  carbon  atoms,  by  the  fact  that  in  the  case  of  the  multi- 
unions  there  is  a  change  in  the  direction  in  which  the  valence 
acts  and  a  consequent  strain  takes  place;  and  by  WunderlichV 
successful  attempt  to  give  to  these  views  a  better  mathematical 
foundation  and  form. 


1  J.  Wislicenus,  Liebig's  Ann.  167,  343. 

2  Ibid.  220,  146. 

3  LeBel,  Compt.  rend.  92,  843:  Lewkowitsch,  Ber.  16,  1568,  Ladenberg, 
Ibid.  19,  2578. 

4  Ibid.  18,  2277. 

5  Configuration  organiscben  Molekule,  Wiirtzburg,  1886. 

65 


MEMOIRS     ON 

The  idea  announced  by  van't  Hoff  and  LeBel,  concerning 
the  isomerism  of  certain  unsaturated  compounds,  such  as 
fu  marie  and  maleic  acids  or  crotonic  and  isocrotonic  acids,  has, 
on  the  contrary,  remained  unfruitful  up  to  the  present  time. 
To  be  sure  the  facts  known  at  that  time  did  not  uncondition- 
ally necessitate  a  geometrical  explanation,  since  the  possibility 
of  a  difference  in  structure  in  these  cases  was  not  completely 
excluded. 

The  hope  of  getting  such  evidence  has  apparently  been  the 
chief  incentive  to  that  important  series  of  investigations  carried 
on  by  Fittig  with  the  help  of  his  students,  which  has  been 
published  in  Liebig's  Annalen  from  the  year  1877  onward. 
Although  these  investigations  have  been  rich  in  important  and 
surprising  results,  they  have,  nevertheless,  so  far  as  their 
theoretical  object  was  concerned,  been  essentially  without 
results. 

Even  though  many  of  the  peculiar  relations  existing  between 
fumaric  acid  and  maleic  acid  may  find  a  satisfactory  explana- 
tion in  Kolbe's  assumption  of  a  bivalent  saturated  carbon  atom 
with  two  of  its  valences  entirely  unengaged,  there  are  certain 
relations  known  between  these  two  acids  which  cannot  be  sat- 
isfactorily explained  by  means  of  the  constitutional  formulas: 
CH.  CO.OH  OHg.CO.OH 

OH.  CO.OH  =C.    CO.OH. 

For  example,  this  formula  does  not  give  the  slightest  infor- 
mation as  to  why  fumaric  acid  when  oxidized  by  permanganate 
gives  raceinic  acid,  while  maleic  acid  on  the  contrary  gives 
inactive  tartaric  acid.1 

Similar  efforts  to  explain  the  isomerism  in  the  case  of  other 
unsaturated  compounds,  show  even  more  completely  the  un- 
satisfactory nature  of  this  explanation2,  and  in  view  of  Fittig's 
important  series  of  investigations  it  must  be  assumed  that 
there  are  isomeric  compounds, — especially  unsaturated  com- 

1  Ber.  13,  2150;  14,713;  and  Liebig  Ann.  226,  191. 

2  In  connection  with  this  should  be  especially  mentioned  the  recent 
revival  of  the  hypothesis  of  Roser  (Ber.  15,  2347.)  by   Anschutz  (Liebig 
Ann.  239,  170).    I  shall  return  to  this  later  in  connection  with   a  dis- 
cussion of  some  investigations  in  the  fumaric  and  maleic  acid  group. 

66 


STEREO-CHEMISTRY. 

pounds, — whose  differences  cannot  be  expressed  by  differences 
ih  the  structural  formulas. 

In  recent  times,  owing  to  the  discovery  of  a  third  and  appa- 
rently a  fourth  monobromcinnamic  acid  formed  by  the  addition 
of  hydrobromic  acid  to  phenylpropiolic  acid,  Arthur  Michael1 
has  reached  the  same  conviction.  In  his  opinion  "it  is  impos- 
sible to  express  this  kind  of  isomerism  by  means  of  our  present 
theories; "  he  seeks  therefore  to  designate  it  at  least  by  a  special 
name,  "  Alloisomerism." 

Shortly  after  this  Erlenmeyer2  stated  that  he  had,  in  com- 
pany with  H.  Stockmeier,  four  years  before  prepared  the  two 
new  monobromcinnamic  acids,  and  had  considered  them  as 
polymeric  modifications  of  the  previously  known  form.  The 
reasons  for  this  assumption  he  does  not  mention,  but  he  has  also 
held  that  other  bodies  considered  as  "  alloisomeric "  by  A. 
Michael  were  merely  polymeric  ;  thus  fumaric  acid  is  for  him 
"certainly  composed  of  two  molecules  of  maleic  acid." 

Erlenmeyer  thus  succeeds  in  giving  a  possible  explanation  to 
apparently  abnormal  cases  of  isomerism,  and  has  avoided  the 
introduction  of  any  new  principle  opposed  to  this  explanation  ; 
but  he  has  done  this  by  the  aid  of  a  speculation  insufficiently 
supported  by  the  facts  or  in  direct  opposition  to  them.  Fumaric 
acid,  at  least  when  it  passes  over  into  racemic  acid  by  oxida- 
tion, is  not  a  polymere  of  maleic  acid  ;  certainly  such  a  relation 
does  not  exist  between  the  esters  which  have  the  same  vapor 
density, — and  that  corresponding  to  the  simplest  possible  molec- 
ular weight, — and  still  show  all  of  the  differences  shown  by  the 
acids. 

Neither  Michael  nor  Erlenmeyer  have  considered  the  possi- 
bility of  a  geometrical  cause  for  this  isomerism,  or  if  they  have 
done  so  they  have  thought  it  to  be  so  objectionable  that  they 
have  made  no  mention  of  it.8  Just  as  I  was  convinced  formerly 
of  the  structural  identity  of  optically  different  compounds,  so 


1  Ber.  19,  1378,  1381. 

a  Ibid.  19,  1936. 

8  After  the  manuscript  of  this  paper  had  gone  to  the  press,  there  ap- 
peared (Ber.  20. 550.)  a  new  work  of  Michael  and  Browne  upon  "Isomerism 
in  the  cinnamic  acid  group  "  in  which  this  question  was  touched  upon, 
but  it  was  dismissed  with  the  remark  that  the  van't  Hoff  theory  was 

67 


MEMO IB S    Otf 

now  I  am  led  to  attempt  to  find  the  cause  of  the  deviation  in 
the  chemical  and  physical  properties  of  such  compounds  in  the 
different  arrangements  in  space  of  the  elementary  atoms  which 
are  linked  together  in  the  same  order  of  succession. 

This  effort  will  in  general  be  successful  if  each  of  the  isomers 
concerned  can  be  assigned,  on  some  plausible  grounds,  formu- 
las with  different  space  relations,  that  are  structurally  identical, 
especially  if  these  formulas  are  in  the  widest  sense  rational, 
that  is,  if  from  them  all  of  the  otherwise  unexplainable  chemi- 
cal and  genetic  relations  follow  as  a  simple  consequence. 

In  the  present  paper  I  have  endeavored  first  to  furnish  the 
proof  that  our  present  experimental  material  is  not  only  com- 
pletely sufficient  to  accomplish  this  result,  but  that  the  attempt 
to  introduce  geometrical  conceptions  leads  to  a  theory  which  en- 
ables us  to  explain  in  a  surprisingly  simple  way  all  of  those 
facts  which  have  hitherto  been  considered  "  abnormal."  If  this 
effort  is  in  fact  successful  I  shall  in  later  publications  make  the 
attempt  to  show  that  the  experimental  investigations  of  some 
of  the  new  problems  arising  from  my  theory  have  confirmed  the 
theoretical  assumptions  in  all  essential  particulars. 

II.   THE  NECESSARY  EXTENSION"  OF  THE  THEORY  REGARDING 

THE  SPACE  ARRANGEMENT  OF  THE  ELEMENTARY  ATOMS 

IN  ORGANIC  COMPOUNDS. 

§  2.  The  conviction  that  most  compound  molecules  must 
occupy  space  of  three  dimensions  is  quite  general,  especially 
will  this  be  true  of  carbon  compounds,  so  far  as  they  contain 
more  than  three  atoms.  In  the  present  condition  of  chemistry 
the  belief  that  the  four  valences  of  carbon  are  all  equivalent, 
meets  with  no  opposition.  The  simplest  geometrical  conception 
corresponding  to  this  fact  is  the  assumption  that  the  direction 
from  itself  in  which  a  carbon  atom  holds  other  atoms,  cor- 


not  sufficient  to  distinguish  between  physical  and  chemical  isomerism. 
The  following  sentence  ("It  appears  to  me  to  be  a  very  doubtful  as- 
sumption," etc.)  shows  that  Michael  considers  the  difference  between 
the  fermentation  lactic  acid  and  the  para  lactic  acid  merely  a  difference 
in  optical  properties,  the  deviation  in  the  chemical  properties,  as  for  ex- 
ample the  salts,  he  entirely  forgets. 

68 


STEREO-CHEMISTRY. 

responds  to  the  direction  of  the  corners  of  a  regular  tetrahedron 
from  its  middle  point. 

Four  like  elementary  atoms  in  combination  with  a  carbon 
atom  are  in  all  probability  situated  in  such  directions  from  the 
carbon  atom.1 

it  is  also  a  generally  recognized  law  that  when  elementary 
atoms  or  atom  groups  are  combined  with  a  carbon  atom  by 
means  of  a  single  valence  they  cannot,  except  under  special 
conditions,  exchange  places  one  with  another ;  since  the  iso- 
mers  which  are  caused  by  the  asymmetry  of  the  carbon  atom  do 
not,  except  under  certain  conditions,  pass  one  into  the  other. 

To  this  law  formulated  and  adopted  by  Baeyer,  it  is  neces- 
sary, for  our  purpose,  to  add  something  more. 

§  3.  When  two  carbon  atoms  are  combined  with  one  another 
with  but  one  valence  and  are  combined  with  other  elementary 
atoms  or  atom  groups,  the  two  resulting  systems  which  are 
united  in  only  one  direction  must  be  capable  of  rotation  about 
their  common  axis.  The  two  systems  rotate,  owing  to  heat 
impulses,  now  in  opposite  directions  and  now  in  the  same  direc- 
tion, but  in  the  latter  case  with  different  velocities  ;  unless  for 
some  special  reason  the  two  systems  remain  fixed  with  respect 
to  one  another,  or  unless  one  position  is  more  favored  than  all 
others  which  would  tend  to  the  production  of  molecular  aggre- 
gates. 

When  there  is  multi-union  between  the  two  carbon  atoms 
the  relation  of  the  two  systems  to  one  another  is  entirely  dif- 
ferent. When  this  is  the  case  the  rotation  of  the  two  systems 
in  opposite  directions  or  in  the  same  direction  with  different 


1  The  configuration  of  the  molecule  ought  to  have  a  decisive  influ- 
ence upon  the  crystal  form  of  chemical  bodies.  The  compounds  of  car- 
bon with  four  elementary  atoms  of  the  same  kind  should  therefore 
crystallize  in  the  regular  system. 

Carbon  tetra  iodide,  C  14,  crystallizes  according  to  Gustavson  (Lie- 
big's  Ann.  172,  175. )  in  regular  octahedra.  Carbon  tetra  bromide,  C  Br4, 
crystallizes  according  to  Bolas  and  Groves  (Liebig's  Ann.  156,  64.)  in 
glistening  tablets,  which  have,  however,  not  been  measured  and  may 
therefore  still  belong  to  the  regular  system.  I  am  at  present  engaged 
in  the  effort  to  prepare  measurable  crystals  of  tetramethylmethane. 
C  (CIl3)4,  in  order  to  get  further  evidence  upon  this  point. 

69 


MEMOIRS     ON 


velocities  is  no  longer  possible;  the  most  that  can  occur  is  an 
independent  oscillation  about  the  common  axis  which  must  now 
lie  in  a  direction  joining  the  two  valences  that  hold  the  carbon 
atoms  together.  The  valences  not  occupied  in  holding  the 
carbon  atoms  together  are  thus  fixed  in  their  relative  positions. 

A  pair  of  doubly  linked  carbon  atoms  have  in  all  four  free 
affinities  for  other  radicals  and,  as  has  been  pointed  out  by 
van't  Hoff,  so  soon  as  the  number  of  different  radicals  present 
is  two  or  greater  they  must  have  besides  the  possible  structur- 
ally different  arrangements  still  two  different  arrangements  in 
space  which  are  structurally  identical. 

To  make  clear  these  relations  we  may  represent  the  systems 
by  tetrahedrons.  Then  in  the  case  of  single  union  between  the 
carbon  atoms  the  tetrahedrons  are  united  by  means  of  a  single 
corner,  e.  g.,  Figure  1.  On  the  other  hand  in  the  case  of  a 
double  union  the  two  tetrahedrons  have  an  edge  in  common, 
and  in  the  case  of  triple  union  they  have  a  face  in  common,  as 
is  shown  in  Figures  2  and  3. 

FIG.  1.                                        FIG.  2. 
//^ ^,H  H, -71s      0  H 

CH3 


CH8 


CH 


§  4.  In  the  latter  case  isomers  are  not  possible;  but  in  the  case 
of  double  union  the  following  opportunities  for  isomerism 
exist. 

(1)     For     C2 


FIG.  6. 


STEREO-CHEMISTRY. 


(2)    For    C2  a2  b  c. 

FIG.  7. 


FIG.  8. 


FIG.  9. 


(3)    Finally  f  or  C2  a  b  c  d  : 


Cab 
n  « 
Ccd 


FIG.  10. 


and 


FIG.  11. 


Cac 

n 

Cbd 


Cad 

n 
Cbc" 


FIG.  14. 


and 


and 


Here  the  three  structurally  different  molecules  eacb  give  two 
different  arrangements  in  space,  so  that  in  all  six  isomeric  com- 
pounds must  exist.1 

§  5.  It  is  not  necessary  to  use  the  picture  formulas  to  express 

l.  For  example  the  two  geometrical  isomers  a-dibrom-  and  /2-dibrom- 
crotonic  acids,  and  the  brommethylacrylic  acids. 

71 


MEM01ES  ON 


these  relations,  but  they  may  be   replaced  by  the  usual  letter 
formulas  if  we  will  agree  in  the  future  to  interpret  by  the  expres- 
sions: 
(1)  a.C.a  a.C.b  a.C.b 


a.C.b 
n 
c.C.d 

n 
b.C.b 

a,C.a 
n 
b.C.c 

a.C.b 
ii 
d.C.c 

n 
a.C.b 

u 
b.C.a 

a.C.d 
u 
c.C.b 

a.C.b 
n 
a.C.c 

a.C.b 
n 
c.C.a 

a.C,c    a.C,c 
n           n 
b.C.d    d.C.b 

a.C.d 
n 
b.C.c 

(3) 


not  only  the  structure,  that  is,  the  order  of  succession  in  which 
the  atoms  are  united  with  one  another,  but  that  these  shall  also 
express  in  the  above  sense  a  different  space  arrangement  of  the 
groups  on  the  two  sides  of  the  axis  of  the  carbon  atom  system, 
and  thereby  understand  that  the  expressions  bracketed  together 
above  are  to  be  recognized  as  different  geometrical  forms. 

The  use  of  the  letter  symbols  in  the  place  of  the  graphic 
formulas  to  represent  the  single  union  of  two  such  systems  is 
somewhat  less  satisfactory,  but  may  be  accomplished  if  in  the 
following  expressions  the  position  of  the  letters  be  taken  to  in- 
dicate the  direction  of  the  valences  from  the  common  axis  of 
the  system. 


FIG.  16. 


Thus 


b 
a.C.c 

a.C.c 


can  take  the  place  of 


and  in  the  case  of  a  rotation  of  one  of  the  systems  with  respect 

FIG.  17. 

bAa  X    ic 

to  the  other  |  can  replace 

a.C.c 
b 


STEKEO-CHEMISTRY. 

§6.  The  number  of  the  stereometric  formulas  developed  in 
this  way  is  entirely  sufficient  to  indicate  constitutional  differ- 
ences in  the  cases  of  all  of  the  abnormal  isomers  ;  it  remains 
only  to  find  some  means  of  determining  which  of  the  possible 
symbols  corresponds  to  each  of  the  known  modifications. 

When  van't  HofE  assigned  to  fumaric  acid  (Figure  18)  and 
inaleic  acid  (Figure  19) 


the  formulas  X~-V  and 


HO.CO.C.H  H.C.COO.OH 

r\v  ^  *\  11  c\ 

H.C.CO.  OH  H.C.CO.OH 

he  was  led  to  the  selection  by  the  fact  that  rnaleic  acid  when 
warmed  easily  passes  over  into  an  anhydride,  therefore  its 
carboxyl  groups  must  be  nearer  one  another  than  in  the  case  of 
fumaric  acid  which  does  not  form  the  corresponding  anhydride 
but  which  when  heated  sublimes  essentially  unchanged. 

In  the  case  of  crotonic  acid  and  isocrotonic  acid,  however,  it 
is  not  so  simple  a  matter  to  distinguish  between  the  two  space 

H.C.CH3  CH3.C.H 

formulas  H.C.CO.OH       and         H.C.CO.OH 

Thus  far  we  have  no  means  of  recognizing  with  certainty 
which  of  the  four  known  monobromcinnamic  acids  corresponds 
to  each  of  the  four  symbols 

H.C.C6H5          C6H5.C.H  Br.C.C6H5        C6H5.C.Br. 

Br.C.CO.OH        Br.C.CO.OH        H.C.CO.OH        H.C.CO.OH 


a-bromcinnamic  acids  /?-bromcinnamic  acids 

These   means   will   be  increased,  nevertheless,  by  following 

with  the  aid  of  models  all  of  the  processes  of  formation  of  the 

unsaturated  geometrically  isomeric  compounds,  as  well  as  their 

transformations  one  into  another,  or  by  following  these  changes 

with  close  attention  to  the  geometric  relations  of  such  changes. 

§7.     When  a  triple  union  betiveen  two  carbon  atoms  passes  over 

into  a  double  union,  if  we   assume  that  the  process  is  one  of 

F  73 


MEMOIRS  ON 


simple  addition  and  that  only  one  of  the  three  pairs  of  valences 
is  affected,  then  it  is  self  evident  that  the  two  radicals  originally 
in  combination  with  the  carbon  atoms  must  be  situated  on  the 
same  side  of  the  common  axis  of  the  two  systems,  that  is, 

FIG.  20.  FIG.  21 . 


or 


+  2c 


4-  2  c  can  give  only 


can  give  only 


a.C.c 
" 

D.vj.C 


so  long  as  the  combinations  of  the  other  two  pairs  of  valences 
is  not  changed.  This  method  permits,  however,  of  only  occa- 
sional application. 

§8.  It  not  infrequently  happens  that  unsaturated  geometric- 
ally isomeric  bodies  are  formed  from  saturated  compounds  by 
the  withdrawal  of  univalent  radicals  from  two  singly  linked 
carbon  atoms  the  valences  of  the  carbon  atoms  thus  set  free 
uniting  to  form  double  union.  The  remarkable  transformation 
from  one  to  the  other  of  structurally  identical  but  different 
modifications,  which  have  never  before  been  considered  from 
this  point  of  view  may  be  analogous  to  this,  in  that  the  com- 
pound at  first  unsaturated  by  the  addition  of  two  radicals  at 
first  passes  over  into  a  saturated  compound  and  then  by  the 
elimination  of  the  same  number  of  radicals  become  unsaturated 
again.  Thus  for  example,  the  transformation  of  maleic  acid 
into  fu marie  acid  by  the  action  of  the  concentrated  halogen 
hydrogen  acids  may  result  from  the  fact  that  the  molecules  of 
the  latter  may  unite  with  the  maleic  acid  to  form  a  monohal- 
ogenated  snccinic  acid  and  that  this,  especially  in  the  presence 
of  water,  again  loses  the  halogen  acid  : 

C;JHa(C0.01I)2-fHBr^C2H3Br.(CO.OH2)=HBr+C2H2(CO.OH)2 
maleic  acid  bromsuccinic  acid  fumaric  acid 

In  this  case  it  can  be  understood  how  small  quantities  of 
concentrated  hydrobromic  acid  acting  as  a  sort  of  ferment 
can  transform  large  quantities  of  maleic  acid  into  fumaric  acid. 

§9.     Among    the   saturated    compounds  there  is  a  possible 


S  T  E  R  E  0  -  C  H  E  M  I  S  T  R  Y  . 

change  of  the  relative  positions  of  the  radicals  in  the  two  systems 
with  respect  to  one  another,  by  a  rotation  of  one  of  the  systems. 
But  if  there  is  a  later  decomposition  of  the  compound  this 
rotation  must  always  give  rise  to  the  same  product,  since  the 
rotation  is  due  to  the  difference  in  the  attraction  that  the  radi- 
cals combined  with  the  different  carbon  atoms  have  for  one 
another. 

If  the  six  radicals  combined  with  such  a  pair  of  carbon  atoms 
are  all  of  the  same  kind  then  the  independent  rotation  of  the 
systems  proceeds  under  essentially  the  influence  of  the  heat 
impulses  alone ;  but  it  is  quite  different  when  the  two  atoms  are 
associated  with  different  radicals.  Here,  to  be  sure,  it  is 
necessary  to  make  an  assumption  ;  namely,  to  assume  that  in  a 
compound  molecule  the  elementary  atoms  not  united  with  one 
another  do  still  exert  an  attractive  influence  upon  one  another, 
not  only  the  influence  of  gravity  but  that  they  exert  an  actual 
chemical  attraction. 

§  10.  On  other  grounds  this  assumption  cannot  be  denied; 
it  is  quite  as  necessary  as  the  assumption  that  chemical  action 
is  due  to  the  chemical  attraction  for  one  another  which  the 
atoms  of  different  molecules  exert  when  they  come  sufficiently 
close  together.  If  this  attraction  did  not  exist  dissociation 
processes  would  be  impossible  ;  dependent  upon  this  are  also 
all  intermolecular  changes,  and  the  entire  group  of  phenomena 
connected  with  the  variation  in  the  substitutive  power  of  single 
elementary  atoms  due  to  the  presence  of  certain  other  atoms, — 
such  as  in  all  the  processes  where  the  first  substitution  exercises 
a  specific  influence  upon  the  chemical  position  taken  in  the 
case  of  a  second  substitution. 

But  the  cause  which  produces  mutual  intermolecular  action 
of  elementary  atoms  not  directly  connected  must  be  the  same 
as  the  cause  which  produces  the  action  of  atoms  in  different 
molecules  upon  one  another ;  it  is  the  action  of  their  specific 
affinities,  tinder  the  influence  of  these  affinities  the  rotation 
of  the  two  singly  linked  carbon  atom  systems  which  are  each 
combined  with  two  different  kinds  of  radicals,  must  take  place 
so  that  the  elementary  atom?!  which  have  the  greatest  affinity 
for  each  other  shall  approach  each  other  as  nearly  as  possible, 
and  the  direction  between  them  will  be  parallel  to  the  axis  of 
the  system. 

75 


MEMOIRS  ON 


Thus,  for  example,  when  ethylene  chloride  is  formed  by  the 
action  of  ethylene  and  chlorine  gases  the  first  configuration  of 
the  molecule  can  be  represented  : 1 


FIG.  23. 


Cla° 


in  consequence  of  the  known  relation  of  the  affinities 

H:H  +  H:H  +  C1:C1<    H  :  II  +  2(  H  :  01  ) 
one  system  will  rotate  with  respect  to  the  other  and  must  go 
over 

FIG.  24.  FIG.  25. 


into 


or 


Cl  Cl 

But  these  two  are  identical. 

§  11.  It  should  be  stated  that  this  position  is  not  an  abso- 
lutely stable  one.  Heat  impulses  of  small  intensity  will  give 
rise  only  to  swinging  of  the  systems  about  the  position  cor- 
responding to  their  most  active  affinities  ;  more  energetic  im- 
pulses, however,  may  overcome  this  direct  attraction  and  as  a 
result  one  system  will  rotate  with  respect  to  the  other. 

In  a  molecular  aggregate  at  sufficiently  high  temperatures, 
there  must  always  exist  some  configurations  which  do  not  cor 
respond  to  the  position  of  greatest  attraction. 

Their  number  will  increase  as  the  mean  temperature  of  the 
mass  increases.  But  the  position  assumed  as  a  result  of  the 
strongest  attractive  force  is  the  most  favorable  position,  and 
even  at  high  temperatures  this  favorable  configuration  is  pres- 
ent in  greater  numbers  than  any  other  configuration  which  may 
be  produced  by  the  heat  impulses. 
1  This  configuration  was  suggested  by  Wunderlich. 

76 


S  T  E  II E  0  -  C  H  E  M I  S  T  R  ir . 


§  12.  One  circumstance  we  must  not  pass  over  here  in 
silence,  even  though  it  is  of  no  special  importance  in  the  ap- 
plication of  the  hypothesis  above  developed  to  known  chemical 
facts,  the  form  of  the  molecule  may  have  an  important  influence 
in  determining  the  crystal  form  of  organic  compounds. 

If  the  four  radicals  combined  with  a  single  carbon  atom  at- 
tract one  another  with  unequal  affinities  the  position  of  the 
valences  of  this  carbon  atom  must  suffer  some  change. 

The  model  of  the  carbon  system  is  then  no  longer  a  regular 
tetrahedron,  as  has  been  pointed  out  by  van't  Hoff1  (see  also 
§58).  Such  a  deviation  will  also  take  place  when  a  similar  rela- 
tion of  the  affinities  exists  between  the  radicals  combined  with 
two  carbon  atoms,  the  carbon  atoms  being  combined  with  one 
another,  in  that  the  pair  of  atoms  with  the  strongest  affinities 
will  tend  to  approach  a  little  nearer  to  each  other.  Thus 


FIG.  26. 


FIG.  27. 


will  tend  to  assume  a  form 
something  like 


§  13.  In  order  to  avoid  needless  repetition  in  the  following 
special  part,  it  is  necessary  that  I  should  bring  forward  here 
one  point  of  general  importance. 

When  an  unsaturated  compound  containing  a  pair  of  doubly 
linked  carbon  atoms  passes  into  a  saturated  compound,  the 
addition  of  the  two  entering  radicals  may  occur  through  the 
breaking  of  either  one  of  the  valences  in  the  double  union 
which  are  geometrically  equivalent.  This  equality  is  complete 
when  each  of  the  carbon  atoms  is  united  with  two  radicals  of 
the  same  kind  so  that  by  the  breaking  of  either  the  one  (1.  V  in 
Figure  28)  or  the  other  (2. 2')  of  the  bonds  will  always  give  rise  to 
geometrically  identical  configurations. 

1  Die  Lagerung  derAtome  im  Raume,  translated  by  Hermann,  Braun- 
schweig, 1877. 

77 


MEMOIRS    ON 


Thus  from 

FIG.  28. 


FIG.  29. 


FIG.  30. 


4.  2  b  we  get 


FIG.  31. 


from 


.4.  2  c  we  get 


and 


and 


The  bodies  represented  by  Figure  29  and  Figure  30  are 
geometrically  identical,  only  that  they  are  observed  from 
different  sides,  the  same  is  true  of  Figure  32  and  Figure  33. 

The  result  is  different  when  at  least  one  of  the  two  doubly 
linked  carbon  atoms  is  combined  with  two  different  radicals 
and  it  becomes  saturated  by  the  addition  of  a  third  radical 
which  is  unlike  either  of  the  other  two.  It  then  becomes 
asymmetric  and  now  there  results,  since  each  of  the  two  points 
of  union  are  equally  favorable  to  the  addition,  a  reverse  order 
in  the  arrangement  of  the  single  radicals,  that  is,  there  arise 
bodies  which  are  not  superposable,  but  one  is  the  reflected  im- 
age of  the  other  : 


FIG.  35. 


FIG.  36. 


+  2  C 


and 


In  Figure  35  the  radicals  taken  in  the  order  a  :  b  :  c  are 
arranged  in  one  direction  (toward  the  left  as  seen  from  the 
center  of  the  system)  ;  and  in  Figure  36  they  are  arranged  in  an 

78 


S  T  E  E  E  0-C  H  E  M  I  S  T  R  Y  . 


opposite  direction  (toward  the  right  when  seen  from  the  center 
of  the  system).  If  now  the  opposite  order  of  arrangement  of 
the  same  radicals  about  an  asymmetric  carbon  atom  causes  op- 
posite optical  activity,  we  have  at  once  an  explanation  of  the 
fact  that  such  reactions  never  give  rise  to  products  which  will  ro- 
tate the  plane  of  polarized  light,  for  the  dextro-  and  Icevo-rotatory 
modifications  are  formed  at  the  same  time  in  equal  quantities. 

§  14.  If  the  passage  from  an  unsaturated  compound  to  a 
saturated  compound  is  followed  by  the  re-formation  of  an  un- 
saturated compound  by  the  splitting  off  of  two  radicals,  then 
the  kind  of  asymmetry  is  without  influence  upon  the  configura- 
tion of  the  product ;  it,  therefore  in  this  case,  makes  no  differ- 
ence which  one  of  the  two  unions  in  the  first  unsaturated 
compound  is  broken  by  the  addition.  Optically  opposite  modi- 
fications give  identical  products  when  they  pass  over  into  unsatu- 
rated compounds,  provided  that  the  radicals  in  combination  with 
the  asymmetric  carbon  atom  and  which  split  off  are  the  same  in 
all  cases. 

If  there  were  an  especial  attraction  between  the  affinities  of 
a  and  c  then  Figure  35  would  give  by  rotation 


FIG.  37. 


and  Figure  36  would  give 


If  now  a  of  the  asymmetric  system  and  c  of  the  other  system 
are  split  off  and  the  resulting  free  valences  of  the  carbon  atoms 
are  united,  from  Figure  37  we  should  get 


FIG.  39. 


FIG.  40. 


and  from  Figure  38  we  should  get 


79 


MEM OIKS  ON 

On  the  contrary  if  the  asymmetric  system  loses  the  radical  c 
and  the  other  system  loses  the  radical  a,  then  Figure  37 


FIG.  42. 


gives     /V\T(        and  Figure  38  gives 


In  Figures  39  and  40  we  can  recognize  at  once  the  identity  of 
configuration;  Figure  41  and  Figure  42  appear,  indeed  at  first 
sight  to  differ,  but  they  are  the  images  of  two  geometrically 
identical  bodies  seen  from  opposite  sides. 

§15.  Later  on,  in  the  discussion  of  unsaturated  compounds 
which  contain  doubly  linked  carbon  atoms,  will  often  occur 
the  necessity  of  referring  to  the  kind  of  symmetry  present. 

In  order  not  to  be  compelled  to  represent  this  graphically  in 
each  case,  it  is  well  to  make  use  of  a  special  name  for  each 
kind  of  symmetry  ;  in  selecting  these  names,  I  have  been  aided 
by  the  advice  of  my  brother-in-law,  Dr.  Th.  Reye,  professor  of 
mathematics  at  the  university  of  Strassburg. 

The  simplest  case  of  geometrical  isomerism  among  unsatur- 
ated compounds  occurs  when  each  of  the  two  doubly  linked 
carbon  atoms  is  united  with  similar  pairs  of  unlike  radicals. 
Such  a  body  with  the  general  formula 

Cab 

n 

Cab 

in  distinction  from  the  structural  isomer 

Caa 

n 

Cbb 

has  long  been  called  symmetrical.  The  positions  of  the  four 
atoms  a  b  in  both  compounds  are  in  fact  geometrically  sym- 
metrical, in  different  ways,  however.  In  the  configuration 

a.C.b 

ii 
b.C.a 

'like  radicals  are   symmetrically  situated  with  respect   to  the 
common  center  of  gravity  and  with  respect  to  the  common  axis 

80 


STEKEO-CHEMISTRY. 
of  both  systems,  their  positions  therefore,  are  center  symmetric 


or  axisymmetric.     In  the  second  configuration 

FIG.  44. 


a.C.b 

ii  or 

a.C.b 


on  the  other  hand  a  :  a  and  b  :  b  are  not  symmetric  with  respect 
to  the  middle  point  or  with  respect  to  the  common  axis,  but 
they  are  symmetrically  situated  with  respect  to  a  plane  pass- 
ing through  the  two  points  of  union  of  the  pair  of  carbon 
atoms  and  vertical  to  the  common  axis.  They  are,  therefore, 
designated  as  plane  symmetric.  The  expressions  center 
symmetric  or  axisymmetric  and  plane  symmetric  may  not  only 
be  used  in  speaking  of  the  radicals  that  occupy  these  positions, 
but  they  are  also  used  for  the  positions  themselves  so  that, 


for  example,  in  a  compound  not 
only  the  relation  of  a':  a  but  also 
b  :  c  can  be  designated  as  center 
symmetric  or  axisymmetric.1 


1  I  shall  use  the  term  center  symmetric  only,  axisymmetry  occurs  also 

a-C-a 

in  the  case     n    but  this  on  the  other  hand  is  just  like  those  that  have 
b-C-b 

been  previously  designated  as  unsymmetrical. 

81 


MEMOIRS  ON 


FIG.  46. 


In 


on  the  contrary  they  are 
plane  symmetric  and  a': 
b  as  well  as  a :  c  are 
axisymmetric.1 


§16.  Positions  which  are  in  a  direction  from  one  another 
which  is  parallel  to  the  common  axis  of  the  two  systems,  will 
hereafter  be  designated  corresponding  positions. 


In 


FIG.  47. 


THE   HYPOTHESIS 


the  positions  a  and  a',  b  and 
b',  c  and  c'  are  correspond- 
ing positions;  in  doubly 
linked  systems  the  plane 
symmetric  positions  are  at 
the  same  time  corresponding 
positions. 


III.     SPECIAL  PART. 

IK   ITS   APPLICATION"   TO    SINGLE    GROUPS   OF 
CHEMICAL   FACTS. 


§17.  This  extension  of  the  theory  concerning  the  space  re- 
lation of  the  elementary  atoms  which  was  developed  in  the 
previous  chapter  was  a  result  of  following  the  forms  in  single 
cases  of  geometrically  isomeric  unsaturated  compounds  by  the 
aid  of  models.  In  what  follows  an  attempt  will  be  made  to 
show,  not  only,  how  completely  this  theory  explains,  from  a 
single  point  of  view,  many  analogous  changes  hitherto  not  un- 
derstood, but  that  it  also  throws  a  clear  light  upon  another 
group  of  chemical  processes  which  have  thus  far  remained 
entirely  in  the  dark. 

In  the  first  place  will  be  taken  up  the  simpler  processes  in 
which  will  be  considered  the  relative  positions  and  the  chemi- 
cal nature  of  the  simple  or  compound  radicals  attached  to  two 
directly  united  carbon  atom  systems;  this  will  be  followed  by 

1  Here  it  is  better  to  use  the  term  center  symmetric  instead  of  axisym- 
metric. 

82 


STEKEO-CHEMISTRY. 

a  consideration  of  processes  in  which  the  relation  existing  be- 
tween three  or  more  systems  have  an  important  significance. 

A.    THE  RELATIVE  POSITIONS  IN  SPACE  OF  THE  ATOMS  IN  DOUBLE 

SYSTEMS. 

1.   THE  UNSATURATED  HYDROCARBONS  AND  THEIR  SUBSTITUTION 

PRODUCTS. 

§18.     The  tivo  Tolandichlorides  and  Tolandibromides. 

The  two  isomeric  tolandichlorides  were  made  at  about  the 
same  time  in  the  year  1871  by  Zinin1  and  by  Limpricht  and 
Schwanert,2  the  former  obtained  the  tolandichlorides  by  boiling 
an  alcoholic  solution  of  tolantetrachloride,  formed  by  the  action 
of  phosphorus  pentachloride  upon  benzil,  with  zinc;  the  latter 
two,  on  the  other  hand,  obtained  the  tolandichloride  by  heat- 
ing stilbene  with  phosphorus  pentachloride  to  170°.  The  re- 
sulting compound  which  is  formed  in  by  far  the  greater  pro- 
portions is  very  easily  soluble  in  alcohol  and  crytallizes  in 
needles  which  melt  at  63°,  while  the  isomeric  compound  formed 
in  much  smaller  amounts  is  soluble  with  much  greater  diffi- 
culty in  alcohol  and  forms  tablets  which  melt  at  153°. 

Liebermann  and  Homeyer8  reached  the  same  results  by  boiling 
with  alcohol  and  zinc  dust  the  tetrachlortolan  which  they  had 
obtained  in  large  quantities  by  conducting  chlorine  into  boiling 
toluene.  Both  of  the  dichlorides  were  farther  obtained  by 
Hanhart4  by  heating  undiluted  benzotrichloride  with  copper 
powder,  and  by  Onufrowicz5  when  he  brought  the  tolantetra- 
chloride which  is  first  formed  by  the  action  of  copper  upon 
benzotrichloride  into  reaction  with  iron  powder  and  acetic 
acid.  In  this  way  he  obtained  for  every  1  part  of  the  tablets 
which  melt  at  153°,  5  parts  of  the  more  easily  soluble  needles 
which  melt  at  63°. 

On  the  other  hand  when  Liebermann  and  Homeyer6  conducted 
chlorine  gas  through  a  chloroform  solution  of  tolan  they 
obtained  only  the  high  melting  tolandichloride  for  which 
they  gave  the  melting  point  143°. 

1  Ber.  4,  288.  4  Ber.  15,  898. 

2  Ber.  4,  379.  6  Ber.  17,  833. 
8  Ber.  12,  1971.                                 6  Ber.  12,  1974. 

83 


MEMOIRS  ON 


All  of  these  facts  which  cannot  be  explained  by  the  present 
theories,  will  from  the  considerations  above  given  be  under- 
stood as  necessary  consequences  of  the  new  hypothesis., 

According  to  §7  if  the  tolandichloride  which  melts  at  143° 
is  a  simple  chlor  addition  product  of  tolan  it  must  be  plane 
symmetric 


FIG.  49. 


+  01 


2= 


Tolan  CeHs.O^O.CeHs  Tolandichloride 

melting  point  143° 

The  isomer  which  results  from  the  withdrawal  of  chlorine 
from  the  tolantetrachloride  must  therefore  be  considered  as 
axisymmetric.  "We  are  led  to  the  same  conclusion  in  regard  to 
this  point  by  the  application  of  the  principle  of  §10  to  tolantet- 
rachloride. The  possible  configurations  for  tolantetrachloride 


are 


FIG.  50. 


FIG.  51. 


FIG.  52. 


CRH, 


of  which  those  corresponding  to  Figure  51  and  Figure  52  are 
more  favorable  positions  than  that  represented  in  Figure  50. 

Both  contain  two  chlorine  atoms  in  corresponding  positions, 
whicli  by  the  coming  together  of  these  positions  —  can  be  given 
up  to  metals.  In  this  case 


FIG.  53. 


FIG.  54. 


Fig.  51  must  give 


and  Fig.  52  gives 


STEKEO-CHEM1STKY. 

that  is,  from  both,  one  and  the  same  center  symmetric  tolandi- 
chloride  (melting  point  68°)  results  as  the  chief  product.  But 
at  the  same  time  there  will  be  with  rising  temperature  an 
increasing  number  of  molecules  with  the  less  favorable  con- 
figuration Figure  50,  (according  to  §11)  so  that  there  are  formed 
also  small  amounts  of  the  plane  symmetric  dichloride,  which 
melts  at  143  °.1 

§  19.  The  tolandibromides  have  thus  far  not  been  prepared 
from  the  tetrabromide,  on  the  other  hand,  Limpricht  and 
Schwanert 2  have  obtained  two  dibromides  by  mixing  an  ethereal 
solution  of  tolan  with  bromine,  namely  : 

(1)  difficultly  soluble  scales  which  melt  at  200°-205o  in  large 
quantities  and 

(2)  easily  soluble  needles  which  melt  at  64°  in  much  smaller 
quantities. 

Liebermann  and  Homeyer3  obtained  the  same  results  by  the 
use  of  a  carbon  bisulphide  solution  instead  of  an  ethereal 
solution. 

The  difficultly  soluble  high  melting  dibromide  is,  here  also, 
plane  symmetric  resulting  from  the  direct  addition  of  the 
bromine.  The  formation  at  the  same  time  of  the  axisymmetric 
dibromide  (melting  point  64°),  is  due  to  the  temporary  exist- 
ence of  tolantetrabrornide  which  is  formed  by  the  addition  of 
bromine  to  the  plane  symmetric  dibromide,  and  which  after 
assuming  the  most  favorable  position  again  gives  up  two  of  its 
bromine  atoms  to  the  tolan  present  : 

FIG.  55.  FIG.  56. 

CKHK 


Melting  point  205° 

1  The  work  of  Blank,  since  published  by  me  in  Liebi^'s  Annalen,  248 
1.,  gives  complete  experimental  confirmation  of  this  law. 

2  Ber.  4,  379. 

8  Ber.  12,  1974. 

85 


MEMOIRS  ON 


FIG.  57. 


FIG.  58. 


by 
rotation— 


— Br; 


melts  at  64° 

In  the  beginning  of  the  addition  of  bromine  to  tolan  only 
the  direct  addition  product  is  formed,  later,  when  the  un- 
changed tolan  molecules  become  rarer,  a  part  of  them  are  car- 
ried over  to  the  tetrabromide  and  this  by  giving  up  half  of  its 
bromine  atoms  to  tolan.  molecules  with  which  it  comes  in  con- 
tact is  transformed  into  the  axisymmetric  isomer.1 

§  20.  Geometrical  isomerism  among  the  unsaturated  hydro- 
carbons of  the  fatty  group  has  thus  far  never  been  observed. 
The  same  is  also  true  thus  far  for  the  halogen  substitution 
products,  with  the  exception  of  the  disubstituted  ethylene.2 

If  the  observations  of  Tawildaroif 8  were  correct,  that  the  de- 
composition of  bromethylenedibromide  with  alcoholic  potassium 
hydroxide  solution  gives  besides  the  unsymmetric  dibromethy- 
lene boiling  at  91°,  an  isomeric  compound  boiling  at  157°,  then 
this  compound  must  be  a  geometric  isomer  of  the  symmetric 
dibromethylene  formed  from  acetylene  which  boils  at  108°-110°.4 
But  without  doubt,  according  to  my  observations  which  are  to 
be  published  in  another  place,  this  compound  boiling  at  157°  is 
essentially  nothing  else  but  bromacetylbromide,  which  is  formed 
from  the  unsymmetrical  dibromethylene  by  the  direct  addition 
of  oxygen.6.  The  vapor  density,  6.97  found  by  Sabanejew  also 
indicates  this,  the  vapor  density  of  dibromethylene  is  only  6.43, 

1  I  have  succeeded  in  getting  complete  experimental  proof  of  this  as- 
sumption.    The  observations  in  opposition  to  it  are  not  entirely  correct. 
The  publication  of  these  results  will  follow  in  Liebig's  Annalen. 

2  The  work  of  Otto  Holz,  Liebig's  Annalen,   250,  230;  and  Puckert, 
Ibid,  page  240,  makes  this  statement  no  longer  true. 

8  Liebig's  Ann.  176,  22. 

4  Sabanejew,  Liebig's  Ann.  178,  116  and  216,  252. 

5  Demole,  Ber.  11,  316. 

86 


STEREO-CHEMISTKY. 

while  the  vapor  density  of  bromacetylbromide  has  almost 
exactly  the  observed  value,  namely  :  6.98. 

On  the  other  hand  there  exist  without  doubt  two  different 
symmetrical  diiodoethylenes,  both  the  products  of  the  action  of 
acetylene  upon  iodine.  Sabanejew1  observed  in  this  reaction 
the  formation  of  a  fluid  as  well  as  a  solid  body.  The  latter 
melts  at  73°  and  boils  unchanged  at  192°,  while  the  fluid  isomer 
is  solid  only  below 0°  ,  and  is  not  volatile  even  with  water  vapor 
without  decomposition.  Hence  the  latter  compound  is  proba- 
bly the  plane  symmetric  acetylenediiodide,  the  product  formed 
by  the  direct  addition  of  two  iodine  atoms  to  acetylene,  for,  ac- 
cording to  the  general  behavior  of  compounds  which  have  iodine 
attached  to  neighboring  carbon  atoms,  such  a  compound  would 
be  very  unstable. 

The  solid  acetylenediiodide,  which  can  be  distilled  un- 
changed, must  be  considered  to  be  the  center  symmetric  com- 
pound with  greater  stability,  it  is  formed  from  the  fluid  diiodide 
by  the  temporary  formation  of  the  tetraiodide  in  a  manner  similar 
to  the  formation  of  center  symmetric  tolandibromide  (previ- 
ous §).  A  thorough  investigation  of  these  relations  is  now  under 
way  in  my  laboratory. 


2.       FUMARIC   ACID,  MALEIC  ACID  AND  THEIR  DERIVATIVES. 

§  21.  THE  extraordinary  and  thusfarunexplainable  relations 
existing  between  fumaric  and  maleic  acids,  are  in  the  light  of 
this  new  theory  no  longer  puzzling.  The  majority  of  the 
phenomena  follow  from  the  theory  at  once  ;  only  a  few  of  the 
known  facts  require  the  assumption  of  intermediate  processes, 
for  the  actual  occurrence  of  which  I  shall,  in  a  special  communi- 
cation, soon  give  the  experimental  proof. 

§  22.  a  The  formation  of  fumaric  acid  and  maleic  acid 
from  malic  acid,  and  the  change  in  the  relative  yields  of  each 
according  to  the  temperature  at  which  the  decomposition  of  the 
malic  acid  takes  place  is  completely  explained  by  geometrical 
considerations  alone. 

If  malic  acid  is  heated  on  an  oil  bath  not  higher  than  150°, 

1  Liebig's  Ann.  178,  118. 

87 


M  E  M  0  I  E  S  ON 


nearly  the  whole  product  is  fu  marie  acid  ;  the  most  favorable 
configuration  (§  11)  corresponding  to  malic  acid  is  : 

FIG.  60.  FIG.  61. 

ill 


CO.  OH 


Maleic  acid  or  its  anhydride  can,  on  the  other  hand,  result 
only  when  the  two  systems  assume  the  less  favorable  position 
shown  in  Figure  62,  with  respect  to  one  another  : 

FIG.  63. 


CO.  Off 


0 


CO.  OH 


in  which  it  is  a  matter  of  no  consequence  whether  first  malei'c 
acid  and  then  its  anhydride  are  formed,  or  first  malic  anhydride 
is  formed  and  then  transformed  into  maleic  anhydride. 


FIG.  64. 


OH 


CO.  OH 


From  the  second  possible  less  favorable  pos- 
ition (Figure  64)  no  unsaturated  dibasic  acid 
can  be  derived.  In  agreement  with  the  point 
of  view  developed  in  §  11  is  further  the  fact 
that,  since  at  higher  temperatures  the  number 
of  molecules  present  in  the  less  favorable  con- 
dition must  be  considerable  greater  than  at 
low  temperatures, — the  greater  the  heat  at  which  the  reaction 
is  carried  on  the  more  there  is  of  the  maleic  anhydride  in  the 
product. 

When  malic  acid  is  heated  up  to  170° — 180°,  the  yield  of 
maleic  acid  anhydride  is  considerable,  but  the  yield  is  essentially 
increased  if  the  temperature  of  the  oil  bath  is  brought  rapidly 
up  to  200°  and  held  at  that  point.  It  never,  however,  reaches 
half  of  the  theoretical  amount,  but  there  always  remains  be- 

88 


STEKEO-CHEMISTKY. 

hind  a  considerable  quantity  of  fu marie  acid,  provided  the  tem- 
perature has  not  been  high  enough  to  volatilize  the  fu  marie 
acid  unchanged  or  to  decompose  it.1 

If  for  any  reason  malic  anhydride  is  formed, — which  can  oc- 
cur only  in  the  case  of  the  less  favorable  configuration  shown 
iii  Figure  62, — the  relative  position  of  the  systems  becomes 
fixed  and  by  farther  decomposition  only  malei'c  anhydride  can 
result. 

Thus,  for  example,  the  latter  alone  is  formed  from  the 
anhydrides  of  acetyl  malic  acid  ;2  and  for  the  same  reasons  the 
anhydrides  of  the  halogen  substitution  products  of  succinic 
acid  yield  the  anhydride  of  maleic  acid  and  its  substitution 
products.8 

§  23.  b  By  the  addition  of  strong  acids — most  rapidly 
with  the  halogen  hydrogen  acids — maleic  acid  is  converted 
almost  quantitatively  intofumaric  acid,4  the  latter,  on  the  other 
hand,  under  the  same  conditions,  remains  unchanged,  or  forms 
with  fuming  hydrochloric  or  hydrobromic  acids  monosubstitu- 
tion  products  of  succinic  acid.  In  this  case  the  mineral  acids 
act  as  a  sort  of  ferment,  and  small  amounts  of  the  mineral  acid  are 
sufficient  for  the  transformation  of  large  quantities  of  maleic 
acid.  In  a  similar  manner  the  esters  of  maleic  acid  are  trans- 
formed into  the  esters  of  fumaric  acid.5 

Owing  to  the  greater  facility  with  which  maleic  acid,  as  com- 
pared with  fumaric  acid,  forms  addition  products,  it  will  first 
take  up  the  elements  of  the  mineral  acid  and  pass  over  into  a 
substitution  product  of  succinic  acid  : 


for  example,      Y\/  +  H  Br 


CO.  Off 


>.OIf 


1     See  my  article  Liebig's  Ann.  246,  91.    2    Anschiitz,  Ber.  14,2791. 

3  Anschutz  and  Bennert,  Ber.  15,  643. 

4  Kekule,  Liebig's   Ann.     Supplement— Bd.    1,     134  ;    Kekule     and 
Strecker,  Liebig's  Ann.  223,  186.     5     Ossipoff,  Ber.  12,  2095. 

G  89 


MEMO  IRS     ON 


in  which  one  of  the  systems  under  the  direction  of  the  stronger 
affinities  rotates  with  respect  to  the  other  and  assumes  the  most 
favorable  position  shown  in  Figure  67 ;  and  now  owing  partly 
to  the  influence  of  the  water  present  and  partly  to  the  greater 
insolubility  of  fumaric  acid,  hydrobromic  acid  is  split  off  and 
fu marie  acid  must  result  : 


FIG.  67. 


FIG.  68. 


EO.CO 


=HBr+ 


CO.  Off 


Fumaric  acid,  on  the  contrary,  by  the  addition  of  hydro- 
bromic acid  would  pass  over  directly  into  the  monobromsuccinic 
acid  with  the  most  favorable  position  shown  in  Figure  67.  In 
this  case  there  is  no  cause  for  rotation  so  that  from  this  addition 
product  only  fumaric  will  be  re-formed. 

§  24.  c  When  the  ester  of  maleic  acid  passes  over  smoothly 
into  the  ester  of  fumaric  acid,1  there  is  probably  first  an  addi- 
tion of  the  iodine,  then  a  rotation  of  the  systems  and  a  splitting 
off  of  hydroiodic  acid,  which  is  followed  by  a  reduction  of  the 
resulting  iodofumaric  acid  : 


FIG.  69. 


FIG.  70. 


after 
rotating^ 


+HI=I2+ 


C2H&.O.CO 


Just  as  in  the  conversion  of  maleic  acid  into  fumaric  acid  by 

iAnschutz,  Ber.  12,  2282. 

90 


STEREO-CHEMISTRY. 


hydrobromic  acid,  the  splitting  off  of  the  latter  enables  it  to  act 
upon  new  amounts  of  maleic  acid  through  the  entire  course  of 
the  reaction,  so  here  will  the  regenerated  iodine  again  enter 
into  the  reaction  thus  converting  large  amounts  of  the  plane 
symmetric  maleic  acid  ester  into  the  axisymmetric  fumaric  acid 
ester.  Just  this  fermentlike  action  of  iodine  and  the  halogen 
hydrogen  acids  is,  moreover,  an  essential  support  to  the  ideas 
above  developed. 

§  25.  d  The  dibromsuccinic  acid,  which  is  formed  by  the 
addition  of  two  bromine  atoms  to  fumaric  acid,  is  completely 
decomposed,  after  boiling  some  time  with  much  water,  into 
hydrobromic  acid  and  brommaleic  acid,1  because  after  the  addi- 
tion (Figure  75)  rotation  must  follow  (Figure  76)  and  then  by 
the  elimination  of  either  of  the  hydrobromic  acid  molecules 
only  the  maleic  acid  derivative  can  result  (Figure  77). 

FIG.  74.  FIG.  75.  FIG.  76.  FIG.  77. 

C0.03 


JLO.C 


CO. OH 


HBr+ 


10.OH   ,£ — '   ~~  CO.OH 

Dibromsuccinic  acid.        Brommaleic  acid. 


70.0H- 

Fumaric  acid. 

§  26,  e  In  a  similar  manner  the  brom  addition  product  of 
malei'c  acid,  isodibromsuccinic  acid,  passes  over  into  brom- 
f u marie  acid.2 

FIG.  78.  FIG.  79.  FIG.  80.  FIG.  81. 

CO.OH      rr  CO.OH       Br  H 

""   1H 


OH    ^ CO.OH 

Malei'c  acid.    Isodibromsuccinic  acid.     Bromfumaric  acid. 
§  27.  f     By  reversing  the  process  dibromsuccinic  acid  is  ob- 


1  Petri,  Liebig's  Ann.    195,  62. 

2  Kekule,  Ann.  Suppl.  2,  91  ;  Ann.  130,  1. 

91 


MEMOIRS     ON 

tained  from  brommaleic  acid  and  fuming  hydrobromic  acid, 
since  Figure  77  +  HBr  leads  to  Figure  >76.  In  -the  same  man- 
ner bromfumaric  acid  passes  over  into  isodibromsuccinic  acid1 
(Figure  81  +  HBr=Figure  80). 

In  addition  to  the  symmetrical  addition  products  mentioned 
there  are  formed  from  the  brommaleic  acid  certain  amounts  of 
bromfumaric  acid  and  from  the  latter  some  dibromsuccinic 
acid.  This  reaction  which  has  to  be  sure  not  been  studied  with 
care  in  all  its  phases  (for  from  the  brommaleic  acid  some  isodi- 
bromsuccinic acid  would  have  been  obtained)  leads  one  to  sup- 
pose that  besides  the  symmetrical  addition  of  hydrobromic  acid 
there  would  be  under  certain  conditions  at  the  same  time  an 
unsymmetrical  addition. 

The  unstable  unsymmetrical  dibromsuccinic  acids  resulting 
in  this  way  from  brommaleic  acid  and  from  tnromf  umaric  acid 
are  identical  and  in  its  three  possible  configurations  there  seems 
to  be  none  that  is  markedly  more  favorable  than  the  others. 
Brommaleic  acid  gives  directly  the  configuration  shown  in 
Figure  82,  bromfumaric  acid  on  the  contrary  gives  directly  the 
configuration  shown  in  Figure  84.  But  both  of  these  can  by 
rotation  take  on  the  configuration  shown  in  Figure  83. 

FIG.  83.  FIG.  84. 


By  the  elimination  of  hydrobromic  acid  Figure  82  will  always 
give  brommaleic  acid,  while  Figure  83  and  Figure  84  will  give 
rise  to  bromfumaric  acid.  Each  of  the  two  unsaturated  brom 
acids  will  give,  rise  to  the  other,  and  hence  by  the  symmetrical 
addition  both  of  the  dibromsuccinic  acids  will  be  formed. 

§  28.  g  The  surprising  fact  that  when  brommaleic  acid  is 
treated  with  insufficient  amounts  of  sodium  amalgam  it  does 
not  yield  maleic  acid,  but  first  gives  fumaricacid2  and  then 
goes  over  into  succinic  acid,  is  without  doubt  due  to  the  fact 
that  this  reaction  does  not  directly  replace  the  bromine  with 


1  Petri,  Annalen  195,  67. 


2  Ibid,  Annalen  195,  64. 
92 


STEREO-CHEMISTRY. 


hydrogen,  but  first  the  hydrogen  is  added  and  a  bromsuccinic 
acid  is  formed  and  this  after  rotation  eliminates  hydrobromic 
acid  : 

FIG.  85.  FIG.  86.  FIG.  87.  FIG.  88. 

CO.  OH    n  CO.  OH      H 


CO.  Off 

*•"• 

§  29.  h  When  brommale'ic  acid  and  bromfumaric  acid  take  up 
two  atoms  of  bromine  they  are  converted  into  the  same  tribromsuc- 
cinic  acid.1  Since  the  latter  contains  an  asymmetric  carbon 
atom  in  the  group  CHBr.CO.OH  it  must  exist  in  two  geometric 

isomers  which  are  optically  opposites,  and  both  of  which  must 
be  formed  in  equal  quantities  from  each  of  the  brom  unsaturated 
acids,  because  each  one  of  the  two  valences  of  the  double  union 
has  the  same  tendency  to  take  up  the  bromine.  Thus  : 

FIG.  89.  FIG.  90.  FIG.  91. 


OR 


CO.  OH 

Brommale'ic  acid. 
FIG.  92. 

HO.  CO 


HO.  CO 


+Br 


CO.  OH 

Bromfumaric  acid. 


VO.OH 


CO.  Off 


and 


O.OB. 


and 


CO.  OH 


But  Figure  91  is  identical  with  Figure  93  and  Figure  90  with 
Figure  94  since  one  of  each  two  is  converted  directly  into  the 
other  by  the  rotation  of  one  of  the  systems. 

§  30.  i  The  fact  that  malei'c  acid  when  oxidized   with   po- 


1  Petri,  Annalen  195, 


93 


MEMOIRS    ON 

tassium  permanganate  solution  gives  inactive  tartaric  acid,1  and 
fu marie  acid  on  the  other  hand  gives  racemic  acid,2  is  quite  as 
strong  proof  of  the  correctness  of  the  geometrical  formulas  as- 
sumed for  f umaric  acid  and  maleic  acid  as  was  the  passage  of 
the  latter  into  the  former  (§  23).  In  both  cases  the  two  carbon 
atom  systems  CH.CO.OH  by  the  addition  of  hydroxyl  pass 
over  into  the  asymmetric  system — CH(OH)  (CO. OH).  When 
formed  from  maleic  acid  the  radicals  attached  to  the  carbon 
atoms  possess  opposite  orders  of  arrangement: 


FIG.  95. 


FIG.  96. 


CO.  OH 


and 


OH 


CO.  OH 


OH 


The  order  of  arrangement  of  CO.  OH  :  OH  :  H  in  Figure  96  is 
for  the  upper  tetrahedron  a  lej:t  handed  one  and  for  the  lower 
tetrahedron  on  the  other  hand  it  is  a  right  handed  one,  so  soon 
as  the  latter  is  brought  into  a  corresponding  position  with  the 
first ;  in  Figure  97  on  the  contrary  the  upper  one  is  right  handed 
and  the  lower  one  is  left  handed. 

There  must  therefore  be  in  each  of  the  molecules  formed  the 
two.  optically  opposite  asymmetric  systems  so  that  only  the  op- 
tically inactive  tartaric  acid  can  result.  Fumaric  acid  on  the 
other  hand  acts  geometrically  quite  differently.  By  the  addi- 
tion of  2.  OH  to  the  pair  of  valences  2. 2'  both  systems  give 
a  right  handed  arrangement,  by  the  addition  to  the  pair  of 
valences  1.  1'  they  give  a  left  handed  arrangement. 

There  thus  arise  from  fumaric  acid  only  molecules  in  which 
the  two  systems  are  alike  in  their  action  upon  the  plane  of  pol- 
arized liglft,  they  are  dextro-rotatory  and  laevo-rotatory,  and  are 
formed  in  equal  amounts,  that  is,  racemic  acid  which  is  capable 
of  being  decomposed  into  the  optically  opposite  modifications 
is  formed. 


1  Kekule  and  Anschiitz,  Ber.  14,  713. 

2  Kekule  and  Anschiitz,     Ber.  13,  2150;  and  Anschiitz,  Annalen,  226,  191. 

94 


8TEKEO-CHEM1STRY. 


Fio.  98. 


FIG.  99. 


FIG.  100. 


HO. 


CO..OH 

Evidently  the  behavior  of  isodibromsuccinic  acid  and  di- 
bromsuccinic  acid  must  be  entirely  analogous. 

§  31.  Of  the  known  transformations  of  bodies  belonging  to 
the  fumaric  and  maleic  acid  groups,  there  are  two  facts,  so  far 
as  our  present  observations  appear  to  show,  that  are  not  in  ac- 
cordance with  the  theory  as  developed.  They  are  the  transfor- 
mation of  a  part  of  the  maleic  acid  in  contact  with  bromine 
into  fumaric  acid1  (§26),  which  at  first  appears  to  be  without 
explanation,  and  on  the  other  hand  the  formation  of  dibrom- 
fumaric  acid  by  the  addition  of  bromine  to  acetylene  dicarboxy- 
lic  acid,2  which  is  in  direct  opposition  to  the  theory  (§  7). 

In  an  extended  series  of  investigations  concerning  which 
I  shall  give  a  full  report  in  another  place,8  it  has  been  shown 
that  in  these  transformations  the  processes  are  not  by  any 
means  so  simple  as  has  hitherto  been  assumed  ;  in  both  cases 
quantities  of  hydrobromic  acid  are  formed  which  are  sufficient 
to  convert  maleic  acid  into  fumaric  acid,  and  brommaleic  acid 
into  bromfumaric  acid  (compare  §23  and  27).  My  experiments 
have  shown  farther  that  the  acetylene  dicarboxylic  acid  forms 
dibromfumaric  acid  only  under  previously  specified  conditions, 
but  on  the  contrary,  in  complete  accord  with  the  theory  it 
gives  dibrommaleic  acid  when  the  action  of  the  hydrobromic 
acid,  produced  as  a  by-product,  upon  the  acetylene  dicarboxy- 
lic acid  is  prevented  (and  indeed  the  larger  the  quantities  the 
better  this  succeeds).  There  exists  at  present,  so  far  as  I 
know,  not  a  single  fact  in  the  genetic  relations  between  fumaric 
acid  and  maleic  acid  which  is  not  clearly  explained  by  these 
geometrical  considerations,  not  one  which  does  not  serve  as  a 
support  for  the  new  hypothesis. 


1  Petri,  Armalen  195,  59. 

2  Bandrowski,  Ber.  12,  2212. 

3  Annalen,  246,  61-91. 


95 


MEMOIRS     ON 


3.     THE  PYKOCITRIC  ACIDS  AND  THEIR  DERIVATIVES. 
§  32.     Of  the  isomeric   unsaturated  acids  which  arise  by  the 
dry  distillation  of  citric  acid  only    citraconic   acid   and   mesa- 
conic  acid  are  geometrically  isomeric  (I.),  while   the   itaconic 
acid  has  its  elements  combined  in  a  different  order  (II.). 

CH3  CH2 

I.     C.  CO.  OH  II.     C.  CO.  OH 

OH.   CO.  OH  CHa.  CO.  OH 

The  latter  kind  of  structure  does  not  permit  of  different  ar- 
rangements in  space,  but  the  former  does.  Citraconic  acid  is 
easily  recognized  as  the  analogue  of  malei'c  acid  as  it  readily 
forms  an  anhydride,  that  is,  it  has  its  two  carboxyl  groups  in  the 
plane  symmetric  position,  while  in  mesaconic  acid  correspond- 
ing to  fumaric  acid  they  are  in  the  center  symmetric  positions. 
§  33.  Citraconic  acid  when  treated  with  mineral  acids  is 
transformed  into  mesaconic  acid.  When  the  elements  of 
hydrobromic  acid  are  added,  a  mutual  rotation  of  the  two 
singly  linked  carbon  atom  systems  takes  place,  so  that  the  nega- 
tive radicals  of  one  system  will  assume  corresponding  positions 
with  respect  to  the  positive  radicals  of  the  other  system.  The 
splitting  off  of  hydrobromic  acid  under  the  influence  of  water 
will  always  lead  to  the  formation  of  mesaconic  acid  : 

FIG.  101.  FIG.  102.  FIG.  103. 


CO.  OH 


CO.  OH 


.CO. OH 


en. 


.  OH 

Citraconic 
acid. 


Monobrompyro- 
tartaric  acid. 
FIG.  105. 


by  rotation  to 

the  more 
favorable  positions 


S  T  E  R  E  0  -  C  H  E  M  I  S  T  R  Y . 

but  by  the  splitting  off  of  hydrobromic  acid  mesaconic   acid  is 
formed  in  both  cases  : 


FIG.  107. 

AYIT_ 

MO.CO, 


CO.  OB1 

Mesaconic  acid. 

On  the  contrary  if  citraconic  acid — or  better  its  anhydride — 
is  covered  with  hydrobromic  acid  which  has  been  saturated  at 
0°,  the  splitting  off  of  the  latter  does  not  occur,  since  there  is 
no  unsaturated  water  present,  and  monobrompyrotartaric  acid 
is  obtained.  In  this  case  two  opposite  asymmetric  modifica- 
tions will  result  according  as  the  acid  breaks  one  or  the  other 
of  the  pair  of  carbon  valences,  and  indeed  both  will  be  formed 
in  equal  quantities.  The  monobrompyrotartaric  acid  obtained 
is  therefore  inactive  but  it  may  be  decomposed  into  a  dextro- 
and  a  laevo-modification.  The  same  two  monobrompyrotar- 
taric acids  are  formed  by  the  addition  of  hydrobromic  acid  to 
mesaconic  acid,  and  in  this  case  the  most  favorable  position 
results  at  once.  Corresponding  to  this  is  the  fact  that  citraconic 
anhydride  and  mesaconic  acid  are  found  to  give  identical  prod- 
uct when  treated  with  fuming  hydrobromic  acid. 

§  34.  If  instead  of  hydrobromic  acid  bromine  is  added  to 
citraconic  acid  and  mesaconic  acid  two  different  dibrompyro- 
tartaric  acids  must  result;  of  these  the  citradibrompyrotartaric 
acid  corresponds  to  the  isodibromsuccinic  acid  and  inactive 
tartaric  acid  the  addition  products  of  malei'c  acid,  and 'the 
mesodibrompyrotartaric  acid  corresponds  to  the  similar  deriva- 
tives of  fumaric  acid,  dibromsuccinic  acid  and  racemic  acid 
(compare  §  25,  26,  and  30). 

Just  as  dibromsuccinic  acid  by  boiling  with  water  passes 
over  into  bromnialeic  acid,  so  mesodibrompyrotartaric  acid 
gives  bromcitraconic  acid  and  by  heating  the  latter  its  anhydride 
is  obtained.  Boiling  water  decomposes  citradibrompyrotartaric 
acid  in  an  entirely  different  manner,  besides  propionicaldehyde, 

97 


MEMOIRS    ON 

carbonic  acid  and  hydrobromic  acid,  brommethylacrylic  acid 
is  obtained.  By  heating  alone  it  gives  bromcitraconic  acid 
anhydride  ;  here  the  heating  causes  the  anhydride  formation 
(citradibrompyrotartaric  acid  anhydride)  first,  and  after  the 
positions  of  the  systems  have  thus  become  fixed  hydrobromic 
acid  splits  off. 

§  35.  The  passage  of  citradibrompyrotartaric  acid  into  brom- 
methylacrylic acid  belongs  to  a  group  of  phenomena  which 
likewise  find  their  complete  explanation  only  by  geometrical 
considerations.  It  is  that  very  common  class  of  decomposi- 
tions, in  which  the  alkali  salts  of  the  halogen  substituted  acids 
split  off  the  metal  bromides  and  carbon  dioxide  and  form  an 
unsaturated  compound, — be  it  a  hydrocarbon,  a  halogen  sub- 
stitution product  of  a  hydrocarbon,  or  an  unsaturated  acid  of 
less  basicity.  Since  these  processes  all  have  something  in  com- 
mon, and  since  following  out  the  space  arrangement  of  the  atoms 
in  their  case  necessitates  the  consideration  of  three,  carbon  atom 
systems,  it  seems  advisable  to  postpone  their  consideration  to  a 
special  chapter. 

4.   THE  UNSATURATED  ACIDS  OF  THE  ACRYLIC  ACID  GROUP. 

§  36.  In  the  group  of  acids  which  have  a  constitution  ana- 
logous to  that  of  acrylic  acid  there  are  numerous  cases  of  iso- 
merism  which  must  be  due  to  a  difference  in  the  arrangement 
in  space  of  the  elementary  atoms  which  are  combined  with  one 
another  in  the  same  order  of  succession. 

Acrylic  acid  itself  as  well  as  its  substitution  products  is  not 
capable  of  forming  geometrical  isomers,  since  in  the  structure 

H.C.H. 

H.O.CO.OH 

there  is  only  one  possible  arrangement  and  the  formulas 

H-O-H  H-C-H 

ii  an(i  ii 

K-C-CO.OH  HO.CO-C-R 

represent  identical  arrangements.  The  /?-  and  ^-substitution 
products  of  acrylic  acid  must,  on  the  contrary,  exist  in  two 
geometrical  isomeric  modifications,  and  they  are  known  in 
great  numbers  among  the  homologues  of  acrylic  acid  and  their 
analogous  aromatic  compounds. 

98 


S  T  E  K  E  0  -  0  H  E  M  I  S  T  K  Y  . 

For  some  time  it  appeared  as  though  the  space  isomer  of 
rnouochloracrylic  acid  had  also  been  prepared,  ior  Otto  and 
Beckurts1  obtained  from  a-dichlorpropionic  acid  a  liquid  boil- 
ing at  176°-180°  which  appeared  to  be,  without  doubt  a-chlor- 
acrylic  acid,  so  that  the  acid  melting  at  64° -65°  obtained  by 
Werigo  and  Werner2  from  a-/3-ehlorpropionic  acid  appeared  to 
be  a  geometrical  isomer  of  the  undoubted  /3-chloracrylic  acid 
melting  at  84° -85°,  and  first  obtained  by  Pinner  and  Bischoft'3 
by  the  reduction  of  chloralide  with  zinc  and  hydrochloric  acid. 
But  later  Otto  and  Beckurts4  recognized  that  their  supposed 
a-chloracrylic  acid  was  really  a  mixture  of  equal  molecules  of 
a-dichlorpropionic  acid  and  pyrotartaric  acid,  and  that  from 
the  former,  by  means  of  dilute  potassium  hydroxide  solution, 
they  prepared  the  acid  obtained  by  Werigo,  and  this  is  believed 
to  be  the  a-chloracrylic  acid. 

Since  /3-chloracrylic  acid  may  be  formed  by  the  addition  of 
hydrochloric  acid  to  propiolic  acid  it  probably  has  a  constitu- 
tion corresponding  to  the  formula  : 

Cl-C-H 

H-C-CO.OH 

The  geometric  isomer  of  this  is  still  lacking, 
(a)  TJie  Crotonic  acids  and  Methylacrylic  acid. 

§  37.  Since  it  has  been  shown  by  R.  Friedrich5  that  /3-chlor- 
isocrotonic  acid  and  chlorcrotonic  acid  by  the  action  of  alkalis 
and  also  by  the  action  of  heat  at  temperatures  which  lie  far  be- 
low the  transformation  temperature  of  isocrotonic  acid  into 
crotonic  acid,  give  exactly  the  same  transformation  product, 
the  assumption  of  a  difference  in  the  structure  of  the  two 
chlor  acids  must  be  given  up. 

Crotonic  acid  and  isocrotonic  acid  both  have  the  formula 
CH3.  CH:CH.CO.OH,  and  their  /?-chlor  substitution  products 
are  CH3.CC1:CH.CO.OH.  For  each  of  these  structural  formu- 
las there  exist  two  arrangements  in  space  : 

1  Ber.  9,  1879;  and  10,  1948. 

2  Liebig's  Annalen  170,  168. 

3  Ibid.     179,  85, 

4  Ber.  18,  240. 

6  Liebig's  Annalen  219,  322. 


M  E  M  0  I II  S     ON 


FIG.  108. 


FIG.  109. 


CO.  OH 


CO.  OH 


Crotonic  acid.       Isocrotonic  acid, 
FIG.  110.  FIG.  ill. 


CO.  OH 


CO.  OH 


/3-Chlorcrotonic  acid      /2-Chlorisocrotonic  acid 
melting  point  94.5  ° .        melting  point  59.5  ° . 

and  similarly  there  will  be  two  a-chlorcrotonic  acids  : 


FIG.  112. 


FIG.  113. 


CII. 


co.  OH 


CO-OH 


Which  of  these  methods  of  arrangement  corresponds  to  each 
of  the  known  compounds  has  been  determined  only  with  some 
degree  of  probability  by  their  relation  to  tetrolic  acid,  the 
transformation  of  one  of  the  isomers  into  the  other  as  in  the 
case  of  maleic  acid  and  fu  marie  acid  has  not  been  accomplished. 

Both  /3-chlorcrotonic  acids  when  treated  with  potassium  hy- 
droxide solution  pass  over  into  tetrolic  acid,  the  /?-chlorcrotonic 
acid  undergoes  this  transformation  in  the  shortest  time  with 
the  smoothest  reaction  with  dilute  caustic  potash  and  at  a  tem- 
perature of  70  01,  while  the  chlorisocrotonic  acid  requires  a  tem- 
perature considerably  over  10002  and  gives  a  much  smaller  yield 
of  tetrolic  acid.  The  configuration  of  /?-chlorcrotonic  acid  must 


Liebig's  Annalen,  219,  347,  349. 
2    Ibid.  341-345. 


100 


S  T  E  R  EO  -  £  H 


therefore  be  better  adapted  to  the  formation  of  tetrolic  acid  ; 
that  is  it  corresponds  to  Figure  110.  For  the  #-chlorisocrotonic 
acid  there  remains  the  space  formula  Figure  111,  for  isocro- 
tonic  acid  Figure  109,  and  for  crotonic  acid  Figure  108.  The 
observation  that  /3-chlorcrotonic  acid  melting  at  94.5°  and  not 
chlorisocrotonic  acid  results  from  the  addition  of  hydrochloric 
acid  to  tetrolic  acid,  leads  to  the  same  result. 

Which  of  the  two  configurations  Figures  112  and  113  corre- 
sponds to  the  known  a-chlorcr*otomc  acid,  with  a  melting  point 
of  97.5,  cannot  be  determined  by  means  of  the  facts  at  hand. 

In  consequence  of  this  difficulty  I  have  undertaken  the  rein- 
vestigation  of  the  halogen  substitution  products  of  the  cro- 
tonic acids,  and  have  found  that  the  a-chlorcrotonic  acid  is 
formed  from  the  isocrotonic  acid  dichloride  by  the  action  of 
alkalis.  The  a-chlorcrotonic  acid  must  therefore  be  the  one 
that  has  the  OHa  and  CO.  OH  in  corresponding  positions  since 
by  the  addition  of  two  chlorine  atoms  to  isocrotonic  acid  a  rota- 
tion of  the  systems  with  respect  to  one  another  must  follow  : 


cn: 


FIG.  114.  FIG.  115.  FIG.  116.  FIG.  117. 

Ctfar^ ^JT     Cl^ =^CH8V 


CO.  OS 


Jscrotonic  acid.   Iscrotonic  acid  dichloride.     a-Chlorcrotonic  acid. 

In  the  same  manner  the  chlor  addition  product  of  ordinary 
crotonic  acid  gives  a  new,  fourth  chlorcrotonic  acid,  which  melts 
at  a  much  lower  temperature  (66. OQ),  and  which  for  similar 
reasons  is  considered  the  a-chlorisocrotonic  acid. 

CH3.C.H 
Ol.O.CQ.OH 

The  results  of-  this  work  as  well  as  the  results  of  another 
work  carried  on  at  the  same  time  upon  the  derivatives  of  the 
a-/?-dibrombutyric  acid  resulting  from  the  isomeric  crotonic 
acids,  will  be  first  published  in  another  place.  [Ann.  248.  281.] 

101 


§  38.  Up  to  the  present  time  only  two  brominated  crotonic 
acids  were  known.  One  melting  at  106.5°  was  obtained  by  Bis- 
choff  and  Guthzeit1  from  propenyltricarboxylic  acid,  also  by 
Michael  and  Norton2  from  the  «-dibrombutyric  acid  by  treat- 
ment with  alcoholic  potash  solution  ;  the  second  was  obtained 
in  a  similar  manner  from  the  a-/3-dibrombutyric  acid,  and  from 
the  crotonic  acid  dibrornide,  as  a  mass  melting  at  92°.  The 
latter  was  obtained  in  the  same  way  by  Korner3as  early  as 
1866,  but  in  spite  of  its  high  melting  point  it  was  considered 
by  him  to  be  identical  with  brommethylacrylic  acid.  Erlen- 
meyer  and  Muller4  obtained  it  later  together  witli  a-bromcro- 
tonic  acid  by  the  same  process.  Finally  it  was  investigated  by 
C.  Kolbe5  and  its  melting  point  found  to  be  90°  and  he  believed 
it  to  be  /3-bromcrotonic  acid. 

The  only  ground  for  this  assumption  is  that  it  is  different 
from  the  a-bromcrotonic  acid.  It  is,  however,  without  doubt 
only  the  geometrical  isomer  of  the  latter,  for  according  to  my 
observations  it  results  from  the  isocrotonic  acid  dibromide. 
Quite  recently  Michael  and  Browne6  have  concluded  that  it  is 
"  allo  "-a-bromcrotonic  acid,  since  the  /? -bromcrotonic  acid 
prepared  by  them  from  tetrolic  acid  and  hydrobromic  acid 
melts  at  94.5°-95°  and  has  quite  different  properties. 

Upon  the  grounds  explained  in  the  previous  paragraph  that 
one  of  the  a-bromcrotonic  acids  which  melts  at  106.5°  is  the 
real  a-bromcrotonic  acid,  while  that  one  which  melts  at  90°,  on 
the  other  hand,  is  the  a-bromisocrotonic  acid  : 

H-C-CHs  CHa-C-H 

Br-C-CO.  OH  Br-C.  CO.OH 

a-bromcrotonic  acid  a-bromisocrotonic  acid 

(melts  at  106.5°)  (melts  at  90°) 

Isomeric  with  crotonic  acid  is  methylacrylic  acid,  which  is 
a-methylacrylic  acid  and  hence  can  exist  in  but  one  form  ;  its 

1  Ber.  14,  616. 

2  Ibid.  p.  1202. 

3  Ann.     137,  233. 

4  Ber.  15,  49. 

5  Journ.  f.  prakt.Chem.  [2]  25,  394. 
e  Ibid.  35,  257. 


S  T  E  II  E  0  -  C  H  E  M I S  T  R  Y  . 

halogen  substitution  products,  however,  must  show  geometrical 
isomerism.  Such  is  apparently  the  case  with  the  brornmethyl- 
acrylic  acid,  melting  at  62°-63°,  obtained  from  citradibrompyro- 
tartaric  acid ;  and  the  isobrornmethylacrylic  acid,  melting  at 
66°,  obtained  along  with  the  former  from  mesadibrompyro- 
tartaric  acid1.  New  investigations  of  these  acids  are  now  in 
progress  (compare  §  53). 

(b)  The  Methylcrotonic  Acids  and  their  Homologues. 

§  39.  According  to  our  present  knowledge  of  tiglic  acid  and 
Me  acid,  which  has  been  recently  much  increased  by  the 
thorough  investigations  of  Fittig  and  his  pupils2,  they  bear  the 
same  relation  to  one  another  that  crotonic  acid  bears  to  isocro- 
tonic  acid  ;  i.  e.,  they  are  geometrically  isorneric.  Thus,  for 
example,  by  boiling  for  forty  hours  angelic  acid  is  completely 
converted  into  tiglic  acid. 

Indeed  our  present  knowledge  of  these  striking  relations  is 
very  meagre,  so  that  a  new  experimental  investigation — which 
is  now  under  way — is  much  to  be  desired.3 

§  40.  Similar  phenomena  have  been  observed  with  the  higher 
homologues  of  the  acrylic  acid  series.  Thus,  for  example,  the 
structural  relations  between  the  dibromcaproic  acid  resulting 
from  the  addition  of  hydrobromic  acid  to  sorbic  acid  and  the 
isodibromcaproic  acid  formed  by  the  action  of  bromine  upon 
hydrosorbic  acid  is  not  yet  sufficiently  clear  to  permit  of 
definite  conclusions  in  regard  to  the  configuration  of  the  unsatu- 
rated  compounds  derived  from  them.  The  bromcaproic  acid 
formed  by  the  addition  of  hydrobromic  acid  to  hydrosorbic 
acid,  by  splitting  off  hydrobromic  acid  yields  isohydrosorbic 
acid4  along  with  caprolactone,  a  transformation  that  belongs  to 
this  class  of  phenomena. 

In  hydrosorbic  acid  CH3.  CH2.  CH2.  OH  :  CH.  CO.  OH  if  the 
propyl  and  carboxyl  groups  have  center  symmetric  positions 
(Figure  118),  then  when  it  is  transformed  into  the  saturated 
bromcaproic  acid  (Figures  119  and  120)  a  rotation  will  take 

1  Fittig  and  Krusemark,  Ann.  206,  16. 

2  Ann.  195,  79,  92,  108. 

3  M.  Puckert,  Ann.  250,  240. 

4  Landsberg,  Ann.  200,  51 ;  and  Hjelt,  Ber.  15,  618. 

103 


M E  M  0 I K  S     ON 


place  so  that  these  two  groups  will  assume  corresponding  posi- 
tions with  respect  to  one  another,,  (Figures  121  and  122)  and  by 
the  elimination  of  bromine  and  hydrogen  the  geometrically 
isomeric  acid  with  the  plane  symmetric  position  is  formed 
(Figure  123): 


FIG.  118. 


FIG.  119. 


->.OH 

Hydrosorbic  acid  (?) 
FIG.  121. 


CTO.OS 


FIG.  122. 


FIG.  123. 


by. 

rotation 
Fig.  H9 
becomes  : 


Fig.  120 
/\      becomes 

<4L4, 


era 
.  OH 


both 
give  by 

/\    elimination 
/  \  of  HBr. 


CO.  OH 


Isohydrosorbic 
acid(?) 

§  40  a.  The  higher  members  of  the  acrylic  acid  series,  as  is 
known,  when  treated  with  mineral  acids,  especially  with  a 
trace  of  nitrous  acid,  pass  over  into  isomers  with  a  somewhat 
higher  melting  point,  and — so  far  as  their  decomposition  by 
alkalis  is  concerned — with  the  same  structure.  These  are  hypo- 
gaeic  acid,  olei'c  acid  and  erucic  acid,  and  they  become  gaei'd- 
inic  acid,  elai'dic  acid  and  brassidic  ;icid. 

Provisionally  there  is  ground  for  considering  this  process 
analogous  to  the  change  of  maleic  acid  into  fumaric  acid  (§  23) 
and  the  change  of  hydrosorbic  acid  into  isohydrosorbic  acid, — 
since  the  nitrous  acid  always  acts  here  as  a  sort  of  ferment,  and 
is  therefore  split  off  again  from  its  first  combinations,  and  so 
may  continue  the  process  with  a  new  molecule.  But  in  this 
case  the  oleic  acid  from  the  fats  would  have  the  space  formula. 

E-C-H 

H-C-CO.OH 

and  the  artificial  isomer  would  have  the  formula 

104 


S  T  E  H  E  0  -  C  H  E  M  I  S  T  R  Y . 

H-C-R 

H-C-CO.OH 
in  which  E  stands  for  the  alcohol  radicals  :     Cis  H2? ;  Cis  Hai  ; 

Cl9  H39. 

(c)  Cinnamic  acid,  Us  Isomers  and  Homologues.1 
§  41.  The  literature  upon  this  subject  was  briefly  referred 
to  in  the  introduction.  The  bromcinnamic  acids  formed  by 
the  addition  of  hydrobromic  acid  to  phenylpropiolic  acid,  the 
one  signalized  by  Michael  and  Norton  and  the  other  known 
earlier,  which  had  some  years  before  been  prepared  by  Erlen- 
meyer  and  Stockmeier  although  they  did  not  publish  their 
observations,  must,  according  to  §  7, — assuming  that  only  a 
simple  addition  of  a  bromine  atom  and  a  hydrogen  atom  has 
taken  place — give  rise  to  a-  and  /3-bromcinnamic  acids  in  which 
the  phenyl  group  and  the  carboxyl  groups  are  in  plane  symmet- 
ric positions  : 

C6II5 

0  H-C-C6H5  BrC-C6H5 

«    m         +     *HBr=       I  Brfi_0(XOH  +  <«-*)     HC-CO.OH 

CO.OH 

Hence  the  already  known  bromcinnamic  acid  formed  by  the 
action  of  alcoholic  caustic  potash  solution  upon  ciunamic  acid 
dibromide  will  have  the  space  formula  : 

Cells-C-H  CcHs-C-Br 

11  and  I' 

Br-C-CO.OH  H-C-CO.OH 

a-bromcinnamic  acid.  /?-bromcinnamic  acid. 

Which  of  the  two  possible  configurations  for  the  cinnamic 
acid  formula  C6  H5.  CH  :  CIL  CO.  OH 

,  H-C-C6H5  ,  TT  C6H5-C-H 

1.  ii  and  11.  ii 

II-C-CO.OH  H-C-CO.OH 

1  Since  the  first  publication  of  this  paper  the  assumptions  made  here 
in  regard  to  structural  isomerism  of  the  halogen  substitution  products 
of  cinnamic  acid  have  been  greatly  shaken. 

Compounds  held  to  be  structural  isomers  now  appear  to  be  geometri- 
cally isomeric  with  the  same  structure.     The  results  of  Michael's  work 
on  this   point  are  in   the  meantime  to  be   still   farther  examined.     A 
number  of  my  pupils  are  now  engaged  with  these  investigations. 
H  105 


MEMOIKS     ON 


corresponds  to  the  known  cinnamic  acid  is  indeed  not  yet  de- 
termined with  absolute  certainty,  but  it  appears  to  be  space 
formula  I.,  since  this  formula  alone  will  give  the  long  known 
bromcinnamic  acid.  If  cinnamic  acid  is, 


FIG.  124. 


the  dibromide  will  then  be  : 


FIG.  125. 


FIG.  126. 


FIG.  127. 


which  by 
/P.      rotation       * 
/\      gives     /\         or 


lir 


CO.  OH 


c6//6 


CO.OH 


Br 


CO.OH 


and  these  by   the   splitting  off  of  hydrobromic  acid  give  the 
bromcinnamic  acids  : 


FIG.  128. 


FIG.  129. 


CO.OH 


If  it  were  possible  to  replace  the  single  halogen  atom  of  the 
bromcinnamic  acids  with  hydrogen,  without  forming  by  addition 
the  phenylpropionic  acid,  then  the  bromcinnamic  acid  pre- 
pared from  cinnamic  acid  would  without  doubt  give  an  iso- 
cinnamic  acid  and  the  bromcinnamic  acid  obtained  from 
phenylpropiolic  acid  would,  on  the  contrary,  give  the  ordinary 
cinnamic  acid.  Upon  these  points  also  new  experiments  have 
already  been  begun  in  my  laboratory. 

106 


STEREO-CHEMISTRY. 

§  42.     Atropic  acid  the  structural  isomer  of  cinnamic  acid 

C6H5.  C=CH2  H-C-H 

i  or  ii 

CO.  OH  C6H5.C.CO.OH 

does  not  permit  of  geometrical  isomerism. 

In  fact  the  a-isatropic  acid  first  discovered  by  Losseni  and 
the  /Msatropic  acid  discovered  by  Fittig2  along  with  the  for- 
mer are  undoubtedly  polymers  of  atropic  acid  and  are  struc- 
turally isomeric  with  one  another,  so  that  they  need  not  be 
considered  here. 

§  43.  The  homologues  of  cinnamic  acid,  namely  the 
phenylcrotonic  acids  should  give  geometrical  isomers,  but  the 
investigations  of  Fittig  and  his  pupils3  did  not  extend  in  this 
direction  ;  they  were  concerned  rather  with  the  resulting  lac- 
tones. 

(d)  Coumarin,  Coumarinic  acid  and  Coumaric  acid. 

§  44.  Conmarin  and  its  homologues,  as  is  known,  dissolve 
in  alkalis  with  the  formation  of  highly  unstable  metal  com- 
pounds, from  which  by  the  action  of  acids,  indeed  even  by  the 
action  of  carbonic  acid,  the  unchanged  coumarin  is  again 
separated.  R.  Williamson*  who  has  prepared  these  metallic 
compounds  found  that  in  composition  they  corresponded  to 
the  formula 

CH.  C6H4.OM 

CH.CO.OM 

It  is  therefore  a  phenol  salt,  in  which  not  only  the  metal  in 
the  place  of  the  hydrogen  of  the  phenol  hydroxyl,  but,  —  as  is 
frequently  the  case  with  the  y-  and  d-oxyacids  —  also  the  metal 
in  the  caboxyl  group  can  be  withdrawn  by  the  action  of  car- 
bonic acid,  with  the  formation  of  lactones  : 

CH.C6H4.ONa  CH.C6H4.OH 


CH.  CO.ONa  OH.CO.OH 

CH.C6H4 
2NaHC03+     »  >  0 


By  long  heating  of  coumaric  acid  with  very  concentrated 
alkali  solutions  or  by  fusing  it  with  caustic  alkalis  this  basic 
metal  salt  goes  over  into  an  isomeric  compound  from  which 

1  Ann.  138,  237.     2  Ann.  206,  35.     8  Ann.  216,  97. 
4  Journ.  of  the  Chem.  Soc.  1875,  850. 

107 


M  E  M  0  I  K  S    ON 

only  the  strong  mineral  acids  can  set  free  the  orthocoumaric 
acid.1  The  latter  does  not  give  the  expected  coumarin  when 
heated,  but  is  transformed  into  this  only  when  the  addition 
product  formed  by  the  action  of  fuming  hydrobromic  acid  is 
decomposed  with  water. a 

Upon  this  observation  Fittig3  remarked  :  "This  reaction  is 
the  more  remarkable  since  the  retransformation  of  orthocou- 
maric acid  into  coumarin  has  hitherto  been  accomplished  in 
but  one  way ;  namely  by  heating  the  acetyl  coumaric  acid  to 
high  temperatures."  "  It  is  difficult  to  account  for  the  change 
in  this  instance  by  the  dehydrating  action  of  the  hydrobromic 
acid,  since  it  is  scarcely  to  be  assumed  that  the  phenol  hydroxyl 
in  the  coumaric  acid  will  be  exchanged  for  bromine." 

These  relations,  as  well  as  the  isomerism  first  observed  by 
Perkin*  and  confirmed  by  Fittig  and  Ebert5  between  the  methyl 
ether  of  orthocoumaric  acid  made  from  the  methyl  ether  of 
salicylic  aldehyde  and  corresponding  derivative  of  coumaric 
acid  made  by  treating  the  metallic  derivatives  of  coumarin  with 
the  alkyl  iodide,  can  be  understood  only  by  the  help  of  geomet- 
rical considerations. 

§  45.  The  extreme  ease  with  which  coumaric  acid  forms  its 
anhydride  coumarin  indicates  that  in  its  space  formula  the  two 
groups  that  acting  upon  one  another  give  the  anhydride  for- 
mation must  be  on  the  same  side  of  the  common  axis  of  the 
double  system, 

CH 

ii     ,     that  is,  must  be  plain  symmetric  as  is  the 
CH 
case  with  the  carboxyl  groups  in  male'ic  acid  and  citronic  acid  : 

FIG.  130.  FIG.  131. 


4-  2  KOH  =  Y^J  4-  H20 


Coumarin.  Coumarinic  acid  salt. 

1  Delalande,  Ann. 45,  334;  Bleibtreu,  Ann.  59, 183;  Fittig,  Ann.  226,  351. 

2  Fittig  Ibid. 

3  Ibid,  page  352. 

4  Journ.  Chem.  Soc.  1877  I.     414  and  418. 

5  Ann.  216,  142  (Compare  also  Ibid  226,  353). 

108 


S  T  E  EE  0  -  0  H  E  MIS  T  ft  Y  . 

The  orthocoumaric  acid  on  the  other  hand,  the  salts  of 
which  will  not  yield  coumarin  by  the  action  of  acids  will  have  the 
two  groups  in  the  center  symmetric  positions.  Its  formation 
from  coumarin  by  heating  with  an  excess  of  caustic  alkali  is 
due  either  to  an  intermolecular  change  or  to  the  addition  of 
the  elements  of  the  alkali  just  as  fumaric  acid1  and  malei'cacid2 
do  when  heated  with  alkalis  for  a  long  time  with  the  formation 
of  malic  acid,  or  as  the  ester  of  fumaric  acid  and  the  ester  of 
malei'c  acid  unite  with  sodium  ethylate  to  form  the  ethylether 
of  malic  acid.3  By  the  addition  of  KOH  to  Figure  131  there 
results  : 

FIG.  132. 


or 


CO.  OK 


from  which  by  the  splitting  off  of  KOH  again  after  the  rotation 
the  salts  of  coumaric  acid  will  result  : 


FIG.  134. 


FIG.  135. 


CO.  OS 


§  46.  That  the  oxyphenyl  group  and  the  carboxyl  group 
occupy  center  symmetric  positions  in  coumaric  acid  is  fully 
confirmed  by  the  fact  that  coumaric  acid  is  converted  into  cou- 
marin by  the  addition  of  hydrobromic  acid  which  is  split  off 
again.  The  addition  is  shown  thus: 


1  Linneman  and  Loydl.    Ann.    192,  81. 

2  van't  Hoff.    Ber.  18,  2713. 

.  8  Purdie,  Journ.  Chem.  Soc.  1881  I,  344  and  Ber.  18  Ref.  536. 

109 


MEMOIRS    ON 


FIG.  137. 


Coumaric  acid. 

under  the  influence  of  the  tendency  to  form  lactoues  a  rotation 
of  the  systems  with  respect  to  one  another  takes  place. 

FIG.  139.  FIG.  140. 


After  this  occurs  the  positions  are  fixed  and  by  the  splitting 
off  of  hydrobromic  acid  coumarin  results: 


FIG.  141. 


The  result  is  the  same  when  the 
bromine  is  added  to  the  a-carbon  atom. 


§  47.  The  a-alkyl  ether  coumaric  acids  formed  by  the  action 
of  alkyl  iodides  upon  the  basic  sodium  salt  of  coumarinic  acid 
(§44)  are  evidently  coumarinic  acid  derivatives 

FIG.  142  FIG.  143. 


CO.ONa, 


CO.ONa 


Sodium  salt  of  a-methyl  ether 
coumaric  acid,  or  better  sodium 
salt  of  methyl  ether  coumarinic  acid, 
110 


STEREO-CHEMISTRY. 

on  the  other  hand  by  the  condensation  of  methyl  salicyl  alde- 
hyde with  sodium  acetate  and  acetic  anhydride  the  /3-methyl 
ether  coumaric  acid,  or  simply  the  methyl  ether  coumaric  acid, 
results: 

FIG.  144.  FIG.  145. 


^CO.OII  II"- CO.  OH 

Methyl  ether  coumaric  acid. 

5      THE  TRANSFORMATION  OF  UNSATURATED  COMPOUNDS  INTO 
GEOMETRICAL  ISOMERS  BY  HEAT. 

§  48.  A  large  number  of  unsaturated  compounds  which 
have  geometric  isomers  corresponding  to  them,  are  converted 
into  these  by  heating  to  a  certain  point  for  a  sufficient  length 
of  time,  without  the  presence  of  any  other  chemical  compound, 
and  the  same  change  takes  place  in  small  quantities  by  a  sort 
of  ferment  action.1 

To  this  class  belong,  for  example,  the  partial  transformation 
of  fumaric  acid  into  maleic  acid  anhydride,  of  mesaconic  acid 
into  citraconic  acid  anhydride,  and  the  halogen  substitution 
products  of  fumaric  acid  into  the  anhydride  of  the  correspond- 
ing derivatives  of  maleic  acid.  Analogous  processes  are  the 
transformation  of  isocrotonic  acid  into  crotonic  acid  at  180° — 
190°,  of  angelic  acid  into  tiglic  acid  at  the  same  temperature, 
and  the  /?-chlorcrotonic  acid  into  the  /3-chlorisocrotonic  acid  by 
heating  for  twenty  hours  at  150° — 160°  .  Also  the  transforma- 
tion of  methyl  ether  coumarinic  acid  into  methyl  ether  cou- 
maric acid  takes  place  in  a  similar  manner. 

These  transformations  are  due  to  intermolecular  changes, 
which  may  be  the  result  of  the  loosening  of  the  unions  by  the 

1  Such  a  change  of  position  appears  to  be  the  rule  at  sufficiently  high 
temperatures.  Thus  each  of  the  geometric  isomers  of  tolandichloride 
and  tolandibromide  are  partly  converted  into  the  other.  Each  of  the 
/?-brom  and  the  /?-chlor  as  well  as  the  a-  and  /3-halogen  substitute  cro-' 
tonic  acids  behave  in  a  similar  manner.  In  another  place  I  shall  discuss 
these  interesting  relations  in  greater  detail. 

Ill 


MEMO  IBS    ON 


action  of  heat  so  that  there  is  an  exchange  of  positions  of  the 
radicals  concerned  and  directly  in  combination  with  the  group 

C 

n  so  as  to  form  more  stable  compounds,  or  at  times  the  double 

union  of  the  two  carbon  atoms  may  be  so  far  loosened  that 
under  the  action  of  energetic  affinities  a  rotation  of  the  systems 
may  take  place  without  the  addition  of  radicals  to  these 
nascent  affinities — and  at  last  the  double  union  is  again  es- 
tablished. 

When,  for  example,  the  isocrotonic  acid  goes  over  to  the 
crotonic  acid  this  change  would  in  the  first  instance  be  repre- 
sented by  the  scheme  : 

FIG.  146.  FIG.  147. 


cn. 


CO.  OH' 


CO.Off 


in  which  the  methyl  and  hydrogen  exchange  their  places;  in 
the  second  case  the  following  figures  will  express  the  inter- 
mediate stages  of  the  transformation  : 


FIG.  148. 


FIG.  149. 


FIG.  150. 


CH. 


OIL 


Loosening 

of  the 

union 

by  heat 


CH.OH 


after 
rotating 


CO.Off 


CO.OH 


FIG.  151. 


Change  of 
position 
of  the  H. 


re-establishment 

of  the 
double  unions 


CO.OH 


CO.  OR 


•  As  a  cause  for  this  change  we  have  the  great  affinity  of  the 
methyl  for  the  negative  carboxyl  group.  In  the  transforma- 
tion of  /3-chlorcrotonic  acid  into  isochlorcrotonic  acid  the 


STEKEO-CHEMISTKY. 

negative  chlorine  atom  to  be  sure  approaches  as  near  as  pos- 
sible to  the  CO. OH — perhaps  under  the  influence  of  the  hy- 
droxyl  hydrogen  atom — which,  when  one  considers  the  space 
relations  of  the  carboxyl  group  itself  in  its  relation  to  the  re- 
mainder of  the  compound,  approaches  quite  near  to  the  po- 
sition 3. 

The  transformation  of  the  acids  with  the  fumaric  acid  con- 
figuration into  those  of  the  maleic  acid  group,  is  due  to  re- 
lations entirely  analogous  to  those  in  the  /?-chlorcrotonic  acid 
and  lies  in  the  tendency  toward  anhydride  formation  which  is 
possible  only  with  acids  that  have  their  carboxyl  groups  in  the 
plane  symmetric  positions.  The  reaction  first  takes  place  in 
these  cases  at  temperatures  lying  above  200°  which  is  a  tem- 
perature above  that  required  for  the  formation  of  maleic  acid 
anhydride  from  maleic  acid, 

If  by  the  action  of  heat  the  double  union  is  so  loosened  that 
rotation  can  take  place,  when  the  carboxyl  groups  come  into 
corresponding  positions — and  it  matters  not  whether  they  come 
into  this  position  through  the  greater  affinities  of  these  groups 
for  one  another  or  only  as  a  result  of  the  heat  impulses — as 
soon  as  the  anhydride  is  formed  the  two  systems  are  fixed  in 
corresponding  positions. 

This  change  of  the  configuration  at  high  temperatures  must 
be  considered  as  entirely  analogous  to  the  undeniable  change  in 
the  constitution  of  chemical  compounds  by  intermolecular 
transformations  ;  as,  for  example,  the  change  of  an  optically 
active  modification  of  an  organic  compound  into  its  optical 
opposite,  or  into  the  inactive  modification. 

B.    THE  SPACE  RELATIONS  OF  THE  ATOMS  WITH  THREE  AND 

MORE  CARBON  SYSTEMS. 

1.  The  simultaneous  elimination  of  metal  halides  and  carbon 
dioxide  from  the  salts  of  the  halogen  substitution  products  of 
organic  acids. 

§  49.  We  owe  to  the  work  of  Fittig  and  his  pupils  a  proof 
of  the  great  frequency  and  a  thorough  study  of  a  reaction 
which  was  first  discovered  by  Kekule1  when  he  discovered  the 
decomposition  of  the  neutral  salt  of  citradibrompyrotartaric 

1  Liebig's  Ann.,  Supplem.  Bd.  2,  98. 

113 


MEMOIRS    ON 

acid  into  metal  bromide,  carbon  dioxide  and  brommethyl 
acrylic  acid,  in  the  course  of  his  investigations  upon  the  un- 
saturated  acids. 

Fittig  saw  to  his  surprise  numerous  other  analogous  decom- 
positions of  the  neutral  salts  of  monobasic  brom-acids,  and  be- 
lieved that  this  reaction  must  be  dependent  upon  the  position 
of  the  eliminated  bromine  atom  with  respect  to  the  carboxyl 
group.  His  assumption  that  both  of  them  were  attached  to  the 
same  carbon  atom,  or  in  other  words  that  the  bromine  was  in 
the  a-position  with  respect  to  the  carboxy],  was  due  to  the 
theoretical  probability  that  two  radicals,  which  strongly  influ- 
enced one  another,  would  occupy  positions  in  the  molecule  as 
close  together  as  possible,  opposed  to  this,  however,  was  the 
generally  accepted  belief  that  the  bromhydrocinnamic  acid 
which  melts  at  137°  was  the  a-acid. 

This  view,  to  be  sure,  makes  it  necessary  in  many  cases  to 
assume  an  intermolecular  shifting  of  the  hydrogen  atoms  along 
the  carbon  chain;  as,  for  example,  the  formation  of  styrene  from 
the  brom-hydrocinnamic  acid.  , 

C6H5  C6H5 

CH2  =  NaBr+C02  +  CH 

CHBr  CH2 

CO.ONa 

Bromhydrocinnamic  Styrene. 

acid. 

Erlenmeyer1  was  the  first  to  introduce  important  testimony 
against  this  interpretation,  which,  however,  Fittig2  did  not  con- 
sider decisive  although  he  admitted  that  Erlenmeyer's  views  in 
so  far  as  the  formation  of  styrene  was  concerned  had  great  ad- 
vantage as  they  made  unnecessary  the  shifting  of  the  hydrogen: 
C6H5  C6H5 

CHBr  =Na  Br  +  C02  +        CH 

i  ii 

CH2  CHs 

CO.ONa 

1  Ber.  12, 1607. 

2  Ann.  200,  87. 

114 


STEREO-CHEMISTRY. 

He  declared  the  question  of  the  constitution  of  the  other 
acids  which  underwent  similar  decompositions  was  still  an 
open  one,  and  turns  in  his  criticism  of  Erlenmeyer's  real 
reasons  to  one  of  the  least  strong  proofs — the  fact  of  the  forma- 
tion of  the  supposed  a, — but  in  fact  the  /3-chlorstyrene  from 
the  dichloracetophenone — and  holds  that  it  is  not  valid.  Now 
this  process  is  exactly  similar  to  the  conversion  of  acetone  into 
/3-chlorpropylene. 

CH3  CH3  CHa  CH3 

CO  +  PC15=  POC13+  COlj  and  CC12  =  HC1  +  CC1 
CH3  CH3  CH3  CH2 

The  supposed  a-chlorstyrene  must  therefore  be  the  /3-chlor- 
styrene  : 

C6H5  C6H5         C6H5  C6H5 

CO  +  PC15  =POC1  -h  CC12  and  CC12  =  HC1  +  CC1 
CH3  CH3          CH3  CH2 

Since  that  .time  some  other  facts  have  become  known  which 
decidedly  prove  that  the  metals  of  neutral  salts  are  eliminated 
with  halogen  atoms  in  the  /3-position. 

Wallach1  next  found  that  the  salts  of  the  /5-dichloracrylic  acid 
formed  by  the  reduction  of  chloralide  with  zinc  and  hydro- 
chloric acid  when  warmed  develop  carbon  dioxide,  and  are 
decomposed  into  metal  chlorides  and  chloracetylene 

CC12  CC1 

ii  in 

CH  =  NaCl+C02+CH. 

CO.ONa 

and  according  to  Otto  and  Beckurts2  chlortiglic  acid  is  formed 
from  dichlordimethylsuccinnic  acid  by  boiling  its  salts. 

But  this  can  occur  only  when  the  eliminated  chlorine  atom 
occupies  the  /3-position  with  respect  to  the  eliminated  carbon 
dioxide  : 

1  Ann.  203,  88. 

2  Ber.  18,  853-856. 

115 


MEMOIKS    ON 

CO.OK 

C.Cl  CHa.C.01 

CH3. 6. 01  — KCl+COa+CIIs.  0 

CO.OK  CO.OK 

Before  this,  however,  and  indeed  in  Fittig's  own  laboratory, 
Petri1  had  found  facts  which  lead  to  similar  conclusions. 

According  to  these  tribrom  succinic  acid  is  converted,  even 
by  boiling  its  water  solution,  into  a  dibrom  acrylic  acid,  which 
can  only  be  the  a-/?-dibrom  acrylic  acid  since  it  is  also  formed 
from  mucobromic  acid. 

Here  also,  then,  the  carboxyl  and  the  bromine  which  are 
eliminated  must  stand  in  the  /3-position  with  respect  to  one 
another: 

CO.OH 

CHBr 

CBr2  =HBr+ 

CO.OH 

The  firm  establishment  of  the  idea  that  in  this  kind  of  re- 
action it  is  the  a-halogen  atom  that  takes  part  is  shown  by  the 
work  of  0.  Kolbe2  already  mentioned,  in  which  the  brompro- 
pylene  boiling  at  59°  and  formed  by  the  decomposition  of 
the  sodium  salt  of  crotonic  acid  dibromide,  was  designated  the 
/?-brompropylene.  But  now  it  cannot  be  doubted  that  the 
longest  known  isomer  boiling  at  47°-48°  and  formed  from  brom 
acetone  (CHaCBrgOHs)  is  the  real  /?-brompropylene,  and  the 
one  boiling  at  59°-60°  is  the  a-brompropylene.  So  that  Kolbe's 
work  also  speaks  in  favor  of  the  view  that  it  is  the  /3-halogen 
atom  that  takes  part  in  these  reactions  : 

CH3 

CHBr  CH3 

CHBr  =NaBr+C02+          OH 
CO.ONa  CHBr 

1  Ann.  195,  70. 

2  Journ.  of  prackt.  Cliem.  [2]  25,  392. 

110 


STEREO-CHEMISTKY. 

There  is  in  fact,  so  far  as  I  know,  not  a  single  existing  case 
which  on  grounds  at  present  plausible  gives  support  to  Fittig's 
views,  so  it  must  be  assumed  that  it  is  entirely  general  that  it  is 
the  /3-position  which  renders  possible  the  reaction  in  question. 
§50.  This  process  which  goes  on  without  the  shifting  of 
the  hydrogen  atom  completely  loses  its  surprising  features 
when  it  is  followed  by  the  aid  of  the  geometric  theory;  it  is 
only  necessary  to  consider  the  space  relations  of  the  third  carbon 
atom. 

If  a  /3-halogen  substituted  acid  ;  as  for  example  the  phenyl- 
Fia.  153.  /3-brompropionic    acid    is   converted   into  its 

alkali  salt,  then  under  the  influence  of  the 
especially  strong  chemical  affinity  between 
the  halogen  and  the  metal  these  elements 
will  approach  one  another  as  nearly  as  possi- 
ble, and  the  arrangement  shown  in  Figure 
153  would  result. 

The  distance  between  the  two  valence 
positions  4  arid  V,  which  are  the  ones 
most  in  question,  is,  when  the  common  axes 
of  the  two  double  systems  I.  II  and  II.  Ill  and  the  two 
valence  positions  all  lie  in  the  same  plane,  only  slightly 
greater  than  that  of  the  corresponding  positions  in  a  double 
system,  the  ratio  being  1.023:1.  On  the  other  hand,  should 
the  affinity  between  the  halogen  and  the  metal  cause  a  bending 
of  the  axes  as  mentioned  in  §12  or  change  the  direction  of  the 
attraction  the  approach  of  4  to  1"  might  be  much  nearer.  Since, 
however,  the  metal  atom  is  not  directly  combined  with  the 
carbon  atom,  but  is  combined  with  it  through  the  oxygen  atom 
the  distance  between  it  and  the  halogen  atom  will  in  any  case 
be  much  reduced,  and  this  will  aid  their  chemical  action  upon 
one  another  and  their  elimination  from  the  molecule.  At  the 
instant  this  occurs  the  nascent  valence  of  the  oxygen  previously 
held  by  the  metal  saturates  itself  with  the  fourth  valence  of 
the  carbon  atom  III  set  free  by  the  breaking  of  the  union  be- 
tween the  valences  4'  and  4",  so  that  carbonic  acid  is  set  free,  and 
at  the  same  time  a  uniting  of  the  free  valences  4'  and  4"  the 
unsaturated  compound — in  this  case  styrene — is  formed.  Fig- 
ure 153  therefore  gives: 

117 


FIG.  154. 


MEMOIRS    ON 


NaBr. 


§  51  The  case  with  which  such  decompositions  take  place 
is  extremely  variable.  Thus  in  the  case  of  the  salts  of  the 
phenyl-/?-brompropionic  acid,  phenyl-a-/?-dibrompropionic  acid, 
the  hydrobromic  acid  and  bromine  addition  products  of  tiglic 
acid,  ethylcrotonic  acid  and  citraconic  acid  this  occurs  even  at 
the  ordinary  temperatures,  while  in  the  case  of  the  salts  of 
the  a-/?-dibrombutyric  acid  it  takes  place  only  slowly  even 
when  heated  on  the  waterbath.  Without  doubt  the  radicals 
in  combination  with  the  carbon  atoms  and  their  influence 
upon  relative  position  of  the  atoms  has  a  great  importance 
here.  In  some  cases  the  theory  here  developed  can  account 
for  this. 

Thus,  for  example,  the  difference  in  the  behavior  of  the  cinna- 
mic  acid  dibromide  and  the  crotonic  acid  dibromide  may  be 
ascribed  to  the  fact  that  the  configuration  best  adapted  to  their 
decomposition  —  as  shown  in  Figure  155  —  is  also  undoubtedly 
the  most  favorable  configuration,  while  in  the  similar  figure 
corresponding  to  the  crotonic  acid  dibromide  —  Figure  156  —  in 
consequence  of  the  strong  positive  nature  of  the  methyl  group 
and  its  action  upon  the  bromine  atom  in  the  ^-position  —  it  will 
tend  to  rotate  to  position  shown  in  Figure  157  which  is  not  so 
well  adopted  for  their  decomposition 


Fig.  155. 


Fig.  156. 


Fig.  157. 


§  52.     Besides  the  splitting  off  of  halogen  acids  or  halogen 
metals  and  carbon  dioxide  from  the  molecule  the  p-substituted 

118 


STEREO-CHEMISTRY. 

acids  and  their  salts  very  frequently  undergo  other  changes  — 
parallel  processes  which  are  worthy  of  special  mention,  since 
they  also  find  an  explanation  in  their  space  relationships. 
Often  by  changing  the  outer  or  chemical  conditions  one  of  the 
sirnultaneou  processes  may  be  greatly  retarded  and  the  other 
increased  so  that  occasionally  an  exact  reverse  of  the  pro- 
portions will  result.  It  has  been  quite  generally  observed  that 
the  elimination  of  carbonic  acid  takes  place  easiest  or  perhaps 
only  in  a  neutral  solution  —  or  at  most  in  a  solution  containing 
the  carbonates  of  the  alkalis  while  the  free  /3-halogen  sub- 
stituted acids,  or  their  salts  in  the  presence  of  an  excess  of 
caustic  alkalis  for  the  most  part  break  down  in  other  ways. 

While,  for  example,  a  solution  of  /?-bromhydrocinnamic  acid 
in  the  presence  of  weak  solutions  of  sodium  carbonate  gives  a  pre- 
ponderating amount  (65$)  of  styreue1 


CHBr  =          NaBr+C02  +  CH 

CH2.  CO.ONa  CH2 

and  only  about  5$  cinnamic  acid. 

C6  H5  C6H5 

2CHBr    -f     NaaC03  =  2NaBr  +  C02  +  H20  +2CH 

CH2.CO.ONa  CH:CO.ONa 

and  the  rest  was  converted  into  phenyl  lactic  acid. 

CG  HS  CeHs 

SCHBr  +  H2  0  4-  Na2C03  =  2NaBr  +  C02  +  2CH.OH 
CH2.CO.ONa  CH2.CO.ONa 

if  the  free  acid  be  heated  with  water  38-40$  of  it  is  converted 
into  cinnamic  acid  and  about  60$  forms  phenyl  lactic  acid,  and 
by  heating  the  salts  of  the  acid  with  alcoholic  potash  solution 
about  half  of  it  forms  cinnamic  acid  and  the  rest  forms  styrene. 

This  is  wholly  due  to  the  entire  difference  between  the  most 
favorable  configurations  of  the  free  acid  and  of  its  salts. 

Among  the  three  positions  of  the  systems  in  the  free  acid 

1  Fittig  and  Binder,  Ann.  195,  136.  137. 

119 


MEMOIRS     ON 

FIG.  159.  FIG.  160. 


CO.  OH 

the  most  favorable  positions  certainly  are  those  represented  by 
Figures  158  and  159  which  by  the  splitting  off  of  hydrobromic 
acid  will  give  cinnamic  acid.  Figure  160,  which  alone  is  the 
one  that  would  permit-  of  the  simultaneous  splitting  off  of 
hydrobromic  acid  and  carbondioxide  'with  the  formation  of 
styrene,  is  far  less  favorable.  These  relations  are  exactly  re- 
versed in  the  case  of  the  sodium  salt.  Finally  if  there  are 
still  free  molecules  of  caustic  alkalis  present,  then  the  bromine 
will  be  frequently  eliminated  by  them,  and  the  hydrogen  atom 
in  the  ^-position  will  split  off  with  the  hydroxyl  group  of  the 
alkali,  before  the  bromine  can  react  with  the  metal  of  the  salt, 
and  double  union  is  established  between  the  residues  with  the 
formation  of  cinnamic  acid.  If  the  halogen  is  once  replaced 
by  hydroxyl — and  it  matters  not  whether  the  replacement  is 
effected  by  water  or  by  alkalies  (in  the  latter  case  Figure  160 
represents  the  configuration) — the  formation  of  an  unsatu- 
rated  compound  is  prevented.  The  parallel  decomposition  of 
dibromhydrocinnamic  acid  rests  on  a  complete  analogy  in  the 
relations. 

The  configuration  of  the  molecule  first  formed  by  the  addition 
of  bromine  to  cinnamic  acid  (compare  §41)  as  shown  in  Figure 
161  is  especially  an  unfavorable  configuration,  on  the  other  hand 
Figure  162  and  Figure  163  differ  but  little,  but  are  both  more 
favorable  than  Figure  161 — they  differ  less  in  their  relative  sta- 
bility than  Figure  159  differs  from  Figure  160. 

FIG.  161.  FIG.  162.  FIG.  163 


CO.  OH 


'O.OH 


STEKEO-CHEMISTRY. 

In  consequence  of  this  much  a-bromstyrenc  is  formed  even 
from  the  free  dibromhydrocinnamic  acid.  After  its  conver- 
sion into  the  neutral  salt  the  latter  is  the  only  product — because 
then  Figure  163  represents  a  configuration  decidedly  more  favor- 
able than  Figure  162.  On  the  other  hand  if  a  bromine  atom  is 
removed  by  an  excess  of  caustic  potash  and  simultaneously  the 
corresponding  hydrogen  goes  out  with  the  hydroxyl  then  Fig- 
ure 162  will  result  in  a-bromcinnamic  acid  and  Figure  163  will 
yield  #-bromcinnamic  acid.1 

According  to  C.  Kolbe  there  is  a  general  similarity  in  regard 
to  the  yield  of  the  products  in  the  parallel  process  of  decompo- 
sition of  the  a-/3-dibromabutyric  acid  formed  from  crotonic  acid 
— when  its  solution  is  heated  in  the  free  condition,  or  in  the 
form  of  its  neutral  suits  or  finally  in  the  presence  of  free  alkali. 
It  has  one  difference  from  the  cinnamic  acid  derivatives  in  that 
in  this  case  no  brompropylene  is  formed. 

This  is  apparently  due  to  the  fact  that  the  alkali  causes  the 
formation  of  bromisocrotonic  acid  before  the  slower  splitting 
off  of  carbon  dioxide  can  occur  to  any  measurable  extent. 
(Compare  §38  and  §51.) 

§53.  The  salts  of  geometrically  isomeric  p-halogen  substi- 
tuted acids  will  in  general  by  the  splitting  off  of  metal  chlorides 
and  carbon  dioxide  give  rise  to  geometrically  isomeric  unsatur- 
ated  compounds. 

Although  I  am  at  present  engaged  in  a  systematic  experi- 
mental proof  of  this  law2,  there  are,  however,  now  known  facts 
in  accordance  with  it. 

The  halogen  addition  products  of  citraconic  and  mesaconic 
acids,  which  belong  to  the  class  of  compounds  that  easily  de- 
compose with  the  development  of  carbon  dioxide,  when  boiled 
with  water  or  even  at  the  ordinary  temperature  when  neutra- 
lized with  soda,  give  the  halogen  substituted  methylacrylic  aci-d. 
From  the  citradibrompyrotartaric  acid  only  that  brommethyl 
acrylic  acid  melting  62° — 63°  is  obtained,  from  the  mesadi- 
brompyrotartaric  acid  in  addition  to  this,  some  of  the  isobrom- 
methyl  acrylic  acid  (melting  point  65° — 66°)  is  also  obtained. 

The  change  of  citraconic  acid  and  citradibrompyrotartaric 
acid  respectively  takes  place  in  the  following  phases  : 

1  See  foot-note  pa<re  105. 

2  Results  since  pubished  in  Ann.  248.p.  297,  305, 307,  322  and  250,  p.  246-249. 


MEMOIRS     ON 


FIG.  164. 


FIG.  165. 


FIG.  16. 


CO. OJt Ctf      '  CO.OH  JVaOCO 


CIT. 


Br  +  C02  4 


>.ONa 


CO.OH 


Citraconic  acid.   Citradibrompyrotartaric    Brommethylacrylic 
acid.  acid. 

The    formation    of    isobrommetliylacrylic    acid    follows   an 
entirely  analogous  process  : 

FIG.  168.  FIG.  169.  FIG.  170.  FIG.  171. 


Mesaconic  acid.    Mesadibrompyrotartaric    Isobrommetliylacrylic 

acid.  acid. 

If  in  the  case  of  the  latter  decomposition  some  brommethyl- 
acrylic  acid  results,  this  is  perhaps  due  to  its  greater  stability, 
that  is,  to  the  preponderating  affinity  of  the  bromine  for  the 
methyl  which  causes  an  intermolecular  exchange  of  position  as 
shown  in  the  following  figures. 

From  Figure  170  at  the  instant  of  the  elimination  of  Na  Br  + 
C02 

FIG.  172,  FIG.  173.  FIG.  174. 


OIL 


by  exchange  of 
position 


Br 


11  11" 

The  propyl  aldehyde  which  arises  in  considerable  quantities 
in  both  of  these  decompositions  is  due  to  the  elimination  of  an 
additional  molecule  of  Na  Br  4-  C02  and  at  the  same  time  taking 
up  the  elements  of  water,  without  the  latter  allylene  would  re- 

122 


STEREO-CHEMISTRY. 


suit,  which  has  been  proven  by  Friedrich1  to  be  present  among 
the  products  of  the  action  of  alkalis  upon  brommethyl  acrylic 
acid.  Figure  172  by  eliminating  Na  Br  4-  COg  becomes  : 


FIG.  175 


CII. 


FIG.  177. 


FIG.  178. 


CII 


CII. 


H  H 

V 
o 


Allylene  Propylaldehyde 

2  The  Formation  of  Lactones  and  the  Anhydrides  of  Bibasic 
Acids. 

§  54.  Among  the  reactions  which  are  to  be  ascribed  to  geo- 
metrical causes  belong  without  doubt  the  characteristic  action 
between  elements  which  occupy  the  so  called  y-position  with 
respect  to  one  another  in  a  saturated  carbon  nucleus.2  When 
such  elements  exert  a  strong  chemical  attraction  for  one  an- 
other the  four  carbon  atom  systems  by  rotation  will  eventually 
assume  the  configuration  shown  in  Figure  179  : 

FIG.  179. 


Sodium  salt  of  y-brombutyric  acid. 

The  positions  in  question  approach  one  another  quite  closely, 
indeed  the  distance  between  them  is  only  0.667  of  the  distance 


1  Ann.  203,  359. 

2  Compare  Hjelt,  Ber.     15,  630. 


123 


MEMOIRS    ON 


between  corresponding  positions  in  a  double  system.  The 
energetic  chemical  action  between  the  radicals  occupying  these 
positions  can  therefore  be  readily  understood.  Thus  for  ex- 
ample the  salts  of  the  y-halogen  substituted  acids  decompose 
even  at  the  ordinary  temperatures,  the  configuration  shown  in 
Figure  179  is  by  far  the  most  favorable  for  these.  The  halogen 
atom  unites  with  the  metal  standing  near  it  and  the  second 
valence  of  the  oxygen  atom  unites  with  the  valence  which 
formerly  held  the  halogen  atom.  The  free  y-hydroxyl  acids  on 
the  other  hand  do  not  all  at  once  decompose  into  water  and 
lactones.  For  the  y-oxybutyric  acid  for  example  the  most 
favorable  configuration  would  be  that  shown  in  Figure  180. 
Only  in  consequence  of  the  heat  impulses — and  this  occurs 
rapidly  when  the  solution  is  boiled — does  the  hydroxyl  group 
of  system  IV.  approach  the  side  nearest  the  acid  hydroxyl  of 
system  III.  and  when  this  occurs  water  is  eliminated  and  the 
position  becomes  fixed. 

If  one  of  the  hydrogen  atoms  in  sys- 
tem I.  is  replaced  by  a  negative  radical, 
as,  for  example,  by  carboxyl  as  in  the 
case  of  itamalic  acid,  the  configuration 
that  leads  to  the  formation  of  lactones  ji\ 
will  result  much  easier  since  the  car- 
boxyl will  cause  the  hydrogen  atom  of 
system  IV.  to  approach  as  near  to  itself 
as  possible. 

Such     y-oxyacids    decompose    at    the 
ordinary   temperature    into    water    and 
lactones,  or  lactone  acids;  itamalic  acid 
for  example  even  by  acidifying  its  solution  gives  at  once  para- 
conic  acid: 


FIG.  180. 


077 


CO.OH 
I       /H 
H— C— C— II 

NDH 
/on 
H— c— c=  o 

i 

H 


CO.OII 

I     /n 

H—  C—  C—  II 


H 


—  C—  C— 


0 


124 


STEKEO-CHEMISTKY. 


thus  by  the   action 
of     heat    would   be         /\ 
partly    converted     /    \ 
into 


Diaterebic  acid  in  like  manner  yields  terebic  acid. 

§  55.  Analogous  to  the  lactone  formation  is  the  anhydride 
formation  of  such  polybasic  acids  as  have  two  carboxyls  attached 
to  neighboring  carbon  atoms,  the  hydroxyl  groups  of  these  then 
stand  in  the  y-position  with  respect  to  one  another. 

The  configuration  leading  to  the  formation  of  anhydride  in 
this  case,  is,  to  be  sure,  not  the  most  favorable,  but  is  the  tem- 
porary result  of  the  action  of  the  heat  impulses.  (Compare  §22.) 

Thus  for  example  succinic  acid  at  the  ordinary  temperature 
would  certainly  be: 

FIG.  181. 


or 


When  this  position  is  once  assumed  and 
the  anhydride  formed  the  system  cannot 
return  to  its  most  favorable  configuration 
but   remains   fixed,    and    can    be   again 
C>H  changed  by  rotation  only  after  the  break- 
ing of  the  ring  by  rehydration. 
OH.      §  56.     The  circumstances  surrounding 
the  formation  of  the  d-lactones  also  find 
their    explanation     in    the   geometrical 
relations. 

When  five  carbon  atom  systems  are 
simply  linked  in  a  chain  and  each  of 
the  two  end  carbon  atoms  holds  a  radial  that  is  attracted  by 
the  other  then  the  corresponding  valence  positions  are  almost 
in  direct  contact.  In  the  changes  of  configuration  as  a  result 
of  the  rotation  of  the  systems  due  to  the  heat  impulses,  this 
configuration  will  occasionally  be  formed  even  though  it  is  not 
the  most  favorable  configuration,  at  higher  temperatures  there- 
fore the  reaction  which  is  dependent  upon  this  particular  con- 
figuration will  take  place,  for  example  the  ester  anhydride 

125 


MEMOIRS     ON 

formation  of  the  d-lactones  will  occur.     This  configuration  is 
made  clear  by  Figure  184. 


FIG.  184. 


It  is  assumed  here  as  in  all  the  other  figures  that  the  four 
valence  positions  bear  the  same  relation  to  the  middle  point  of 
the  carbon  atom.  In  this  case  all  of  the  points  of  union  be- 
tween the  carbon  atoms  and  the  two  valence  positions  of  the 
end  carbon  atoms  concerned  in  lactone  formation  lie  in  a  circle, 
where  radius  is  to  the  distance  of  the  middle  point  from  the 
valence  position  (Oi,  :d)  as  1,414:1.  The  other  distances  which 
will  come  under  consideration  have  the  following  values: 


DISTANCE  OF 

Valence  position  to  middle  point  of  car- 
bon atom 

The  valences  in  a  single  system 
Between  middle   points  of    the  carbon 

atoms 

Corresponding  valences  in  double  system 
"  "         "  triple 

"  quadruple  " 
"  "         "  pentuple     " 


e.  g.  line 

A 

B 

c 

D 

Ci.a 

1,000 

0,612 

0,500 

0,375 

a.b 

1,633 

1,000 

0,817 

0,612 

Ci.Cii 

2,000 

1,225 

1,000 

0,750 

b.d 

2,667 

1,633 

1,333 

1,000 

c.d 

2,724 

1,661 

1,361 

1,022 

c.e 

1,779 

1,089 

0,890 

0,667 

c.f 

0,181 

0,111 

0,091 

0,068 

The  two  valence  positions  under  consideration  are  in  fact  so 
near  together  there  then  would  be  no  room  in  the  intermediate 
space  cffor  an  oxygen  atom  united  to  the  two  valence  positions. 
The  distance  of  the  valence  position  of  a  carbon  atom  from  its 
middle  point  in  the  construction  of  the  figure  must  not  be 
thought  of  as  the  distance  to  the  middle  point  of  atoms  united 


S  T  E  R  E  0  -  C  H  E  M  I S  T  R  Y  . 

with  it,  but  is  only  the  distance  to  the  surface  of  such  a  carbon 
atom.  The  actual  distance  between  the  middle  points  of  two 
directly  united  carbon  atoms  is  in  fact  twice  as  great  (CiCn). 
Even  if  the  size  of  the  elementary  atoms  is  not  in  all  cases  the 
same  it  is  not  probable  that  the  carbon  atom  and  the  oxygen 
atom  differ  so  much  from  one  another  as  c  f  from  b  c.  It 
follows  therefore,  that  the  positions  of  the  five  carbon  atoms  of 
a  (5-lactone  cannot  retain  the  position  shown  in  Figure  184,  but 
when  0  enters  between  the  position  c  and  f  either  the  circle 
is  forcibly  opened  or  the  positions  a,  b,  c,  d,  and  /,  leave  the 
plane  of  the  circle  partly  in  one  direction  and  partly  in  the 
other.  In  the  formation  of  the  y-lactone  thus  shown  within 
the  molecule  accompanied  by  an  increase  in  the  curvature  of 
line  joining  the  valence  position  does  not  occur.  Since  the  in- 
termediate space,  c  e,  into  which  the  oxygen  enters,  is  only 
slightly  greater  than  that  between  the  valence  positions  of  a 
single  carbon  atom  so  that  the  entrance  causes  a  very  slight 
disturbance  of  the  ring. 

Perhaps  the  fact  that  the  complete  conversion  of  the 
rf-oxyfatty  acids  into  the  J-lactones  is  considerably  more  diffi- 
cult than  the  conversion  of  the  y-oxyacids  into  the  y-lactones1 
is  dependent  upon  these  relations. 

§  57.  The  entire  difference  in  the  behavior  of  the  ft-  and 
a-oxyacids  can  also  be  readily  understood  by  the  aid  of  these 
geometrical  considerations. 

The  /3-oxyfatty  acids  do  not  decompose  'at  100°  but  at 
higher  temperatures  they  decompose  with  the  formation  of  un- 
saturated  acids — provided  the  a-carbon  atom  is  in  combination 
with  a  hydrogen  atom. 

The  most  favorable  configuration  will  be,  for  example,  in 
the  case  of  the  /3-lactic  acid  that  is  shown  in  Figure  185  ;  un- 
der the  influence  of  heat.  Configurations  corresponding  to 
Figure  186  will  also  be  produced. 

Even  though  the  distance  between  the  hydroxyl  positions  of 
systems  I.  and  III.  in  Figure  186  is  not  much  greater  than  that 


1  Ann.  216,  135. 

127 


MEM  OIKS     ON 


FIG.  185. 


FIG.  186. 


0/7 


between  the  hydroxyl  of  system  I.  and  the  corresponding  hy- 
drogen atoms  of  system  II.  Figure  185.  (1,022:1,000).  Neverthe- 
less it  acts  to  prevent  the  lactone  formation  with  its  consequent 
ring  closing,  since  the  uniting  of  the  two  systems  by  means  of 
an  oxygen  atom  would  result  in  a  considerable  contraction  of  the 
ring  and  the  condition  of  strain  thus  introduced  would  con- 
siderably decrease  the  stability  of  the  molecule.  The  passage 
of  the  /3-lactic  acid  into  the  acrylic  acid  is  favored  by  the  greater 
nearness  of  the  corresponding  positions  (OH  in  system  I.  and 
H  in  system  II.  Figure  185)  and  by  the  undeniable  tendency  to 
react,  that  is,  a  loosening  of  the  union  of  the  hydrogen  atom 
which  occupies  an  a-position  with  respect  to  a  carboxyl  group. 
The  fact  that  the  a-oxyacids  do  not  in  a  similar  manner  give 
rise  to  unsaturated  acids  is  due  to  their  structure;  the  fact  that 
the  formation  of  lactones  and  the  consequent  ring  closing  is 
impossible  has  the  same  cause  as  with  the  /?-oxyacids  but  the 
cause  is  even  more  potent  in  this  case.  Although  in  the  double 
system  the  distance  between  corresponding  positions  is  only  a 
little  less  than  in  a  triple  system,  the  entrance  of  an  oxygen 
atom  attached  to  both  of  these  positions  would  produce  greater 
bending  of  the  axes  or  direction  of  the  valence  directions,  and 
a  consequent  intermolecular  strain  because  the  bending  is  dis- 
tributed between  but  two  carbon  atoms  instead  of  three.  Ester- 
ification  between  two  or  more  molecules,  as  in  the  formation  of 
lactide,  can  just  as  readily  take  place  as  esterification  between 
separate  acid  and  alcohol  molecules. 


128 


STEREO-CHEMISTRY. 

IV.      CONCLUSIONS. 

§58.  Geometrical  considerations  as  to  the  molecular  consti- 
tution can  be  applied  with  good  results  to  still  other  groups 
of  changes  just  as  they  have  been  used  to  explain  phenomena 
in  the  preceding  chapters.  I  pass  over  the  consideration  of 
these  for  the  time  so  that  the  present  paper  shall  not  grow  to 
too  great  proportions.  For  this  reason  I  have  found  it  impos- 
sible to  discuss  exhaustively  all  the  facts  at  hand,  more  of  this 
can  be  done  later,  here  have  been  developed  only  the  chief 
features  of  the  theory  and  their  application  to  the  explana- 
tion of  phenomena  which  have  not  been  previously  understood. 
Later  opportunity  will  be  found,  especially  in  the  communica- 
tion of  the  results  of  experimental  investigations,  to  make  good 
this  deficiency.  The  space  relations  in  ring  formation  I  have 
considered  only  so  far  as  was  necessary  in  the  formation  of  lac- 
tones  and  in  the  formation  of  anhydrides  of  bibasic  acids,  and 
have  purposely  neglected  to  follow  them  farther  since  A.  von 
Baeyer  is  at  present  engaged  with  their  study. 

The  statement  of  von  Baeyer1  that :  "ring  formation  is  appa- 
rently the  phenomena  which  will  give  the  most  information  con- 
cerning the  space  arrangement  of  the  atoms,"  seems  not  to  be  en- 
tirely true.  I  believe  it  to  be  of  far  greater  importance  to 
consider,  as  I  have  done  in  the  preceding  pages,  the  simplest 
changes  in  the  transformation  of  a  saturated  into  an  uusatu- 
rated  body  and  an  unsaturated  into  a  saturated  body,  in  which 
only  two  or  three  carbon  atom  systems  need  be  considered,  and 
in  which  the  question  of  strain  and  the  consequent  change  in 
the  direction  of  the  valences  does  not  enter.  So  valuable  an 
idea  as  this  one  put  forth  by  von  Baeyer  has  already  been  fruit- 
ful and  will  be  more  so  in  the  future,  but  still  great  care  must 
be  exercised  in  its  application.  It  is  also  much  in  need  of  some 
extensions. 

Forcible  alterations  in  the  valence  directions  may  occur  for 
example  in  the  case  of  a  simple  carbon  atom  system,  if  it  holds 
two  different  kinds  of  radicals. 

The  agreement  of  the  four  valence  positions  with  the  four 
corners  of  a  regular  inscribed  tetrahedron  is  possible  only  when 


l  Ber.  18,  2277. 

129 


MEMOIRS    ON 

the  four  attached  simple  or  complex  radicals  are  completely 
alike,  the  chemical  attraction  which  they  have  for  one  another 
must  be  exactly  the  same,  as  in  the  case  of  marsh  gas  CEU  or 
perchlormethane  CCU.  But  if  the  carbon  atom  is  in  combina- 
tion with  three  atoms  of  one  element  (a.a.a.)  and  one  atom  of 
another  element(^)  then  in  consequence  of  the  difference  be- 
tween the  affinity  of  b  for  each  of  the  atoms  a  and  the  affinity 
of  the  atom  a  for  one  another,  the  valence  positions  will  cor- 
respond to  the  corner  of  an  equilateral  three  sided  pyramid 
which  will  be  blunt  if  the  affinity  of  b  :a>a:a  and  on  the  other 
hand  will  be  sharp  if  the  relations  are  reversed. 

When  a  carbon  atom  is  combined  with  a  a  and  ft  b,  if  the 
affinity  of  a  :  a  equals  the  affinity  of  b :  b  and  the  affinity  of  a : 
b  is  greater  or  less  than  the  affinity  of  a :  a  or  the  affinity  of  b : 
b,  then  the  valence  positions  are  to  one  another  as  the  corners 
of  a  quadratic  sphenoid.  Every  other  relation  in  the  relative 
strengths  of  the  affinities  as  well  as  every  increase  in  the 
number  of  kinds  of  atoms  in  combination  must  lead  to  forms 
which  are  farther  and  farther  removed  from  the  form  of  the 
regular  tetrahedron  as  the  number  of  different  relations  among 
the  affinities  increases.  With  every  such  change  in  the  valence 
position  there  must  be  a  deviation  in  the  direction  of  attraction 
from  the  middle  point  of  the  carbon — as  compared  with  the 
normal  direction,  and  thereby  there  is  not  only  a  change  in 
the  form  of  the  system,  but  there  enters  into  the  compound  a 
condition  of  strain.  The  influence  of  the  geometric  relations 
upon  the  properties  of  organic  molecules  is  therefore  extremely 
complicated,  and  for  the  most  part  cannot  at  present  be  de- 
termined. 

This  statement  of  von  BaeyerV,  as  is  the  case  with  all  such 
attempts,  cannot  have  the  value  of  a  theory  which  has  been 
confirmed  by  experience. 

§  59.  It  is,  as  I  believe,  quite  different  with  those  discussions 
which  have  been  given  in  the  second  part  of  this  paper.  They 
have,  in  their  limitation  to  the  simplest  space  relations,  in 
spite  of  the  existing  uncertainty  in  their  application  to  many 
special  cases,  the  value  of  an  actual  theory  which  not  only  gives  a 

l  Ber.  18,  2281. 

130 


STEKEO-OHEMISTRY. 

simple  explanation  to  a  large  number  of  facts  that  have  before 
been  wholly  without  explanation,  but  it  is  also  capable  of 
farther  experimental  proof. 

Of  the  numerous  problems  arising  from  this  theory  a  con- 
siderable number  I  have  undertaken  to  solve  and  many  more 
have  been  undertaken  by  my  pupils. 

Up  to  the  present  so  far  as  definite  results  have  been  obtained 
they  correspond  to  the  theoretical  assumptions  in  all  respects. 

I  have  succeeded,  as  already  mentioned  (§  31)  not  only  in 
removing  the  apparent  contradiction  which  was  offered  by  the 
behavior  of  fumaric  and  imilei'c  acid  and  their  substitution 
products  in  some  of  their  transformations,  but  I  have  obtained 
from  the  chlor  addition  product  of  crotonic  acid  the  second  a- 
chlorpropylene  and  the  fourth  chlorcrotonic  acid  exactly  as 
my  theory  had  indicated,  and  have  prepared  from  the  isocro- 
tonic  acid  dichloride  in  addition  to  the  ordinary  a-chlor- 
propylene  the  already  known  a-chlorcrotonic  acid.  My  pupils 
have  already  proved  that  isocrotonic  acid  dibromide  by  splitting 
oif  hydrobromic  acid  yields  the  a-bromcrotonic  acid  melting 
between  106°-107°  as  was  assumed,  and  that  the  bromcrotonic 
acid  melting  at  90° -92°  formed  from  crotonic  acid  dibromide 
is  actually  the  a-bromisocrotonic  acid.  Further  the  second 
geometric  isomer  brompseudobutylene  and  crotonylenedi- 
bromide  have  been  obtained  by  the  method  foreseen  from  the 
theory,  etc.  In  consequence  of  this  I  dare  even  now  assert 
that  the  theory  has  been  confirmed  by  experiment,  and  I  shall 
have  numerous  opportunities  later  and  in  other  places  to  give 
the  facts  leading  to  this  conclusion. 

§  60.  The  most  important  result  of  the  theory  is  the  com- 
plete clearing  up  of  the  cases  of  so  called,  "  abnormal  "  isomer- 
ism,  and  the  proof  that  the  isomeric  unsaturated  compounds 
may  have  their  relative  space  relations  determined  as  the  result 
of  actual  experimented  investigations.1 

For  the  present  there  exist  practically  no  facts  referring  to 
isomerism  that  are  not  theoretically  understandable. 

If  in  the  future  the  name  "  alloisomerism  "  is  still  used  we 
will  understand  by  it  structural  identity  with  a  different  order 

l  Compare  especially  §  18,  19,  23,  30,  33,  36,  41. 

131 


MEMOIRS     ON     S  T  E  II E  0  -  0  H E  M  I  S  T  R  Y  . 

of  arrangement  in  space  of  the  atoms  in  the  molecule.  I  have 
already  in  the  year  1873  suggested  for  this  the  name  "  geomet- 
ric isomerism  "  *  and  believe  it  to  be  better  because  it  is  more 
significant. 

BIOGRAPHICAL  SKETCH. 

JOHANNES  WISLICENUS  was  born  at  Kleineichstadt  June  24, 
1835.  He  studied  chemistry  at  Halle  and  Zurich.  He  was 
professor  at  the  Polytechnikum  in  Zurich  from  1860-1872,  and 
at  the  University  at  Wiirtzburg  from  1872-1885  ;  since  1885  he 
nas  been  professor  of  chemistry  at  the  University  at  Leipzig. 
His  work  has  been  mainly  devoted  to  organic  chemistry  and 
has  been  very  instrumental  in  the  development  of  this  branch 
of  the  science.  His  work  on  para  lactic  acid  convinced  him  that 
its  structure  was  the  same  as  the  ordinary  lactic  acid,  and  he 
believed  the  difference  between  them  to  be  due  to  a  difference 
in  the  arrangement  of  the  atoms  in  space.  This  work  led  van't 
Hoff  to  his  theory  of  the  asymmetric  carbon  atom.  The  work 
of  Wislicenus  has  done  more  than  that  of  any  other  one  man 
to  bring  stereo-chemistry  into  the  prominent  position  it  now 
occupies. 

As  a  teacher  of  chemistry  Wislicenus  has  always  been  one  of 
the  most  inspiring  and  at  the  same  time  one  of  the  most  love- 
able  of  all  the  great  German  masters  of  chemistry. 

l  Ann.  167,  345. 


132 


BIBLIOGRAPHY. 

(a)  Stereo-chemistry  of  Carbon. 
PASTEUR,  Louis. 

1848-58.  On  the  relations  between  composition,  optical 
activity  and  crystalline  form.  The  tartrates.  Ann.  Chim. 
phys.  [3]  24,  442;  28,  56;  31,  67;  38,  437;  Comptes. 
rend.  28,  477;  31,  480  ;  35,  176  ;  37,  110,  162  ;  46,  615. 

1860.  "  Doubling "    by   means   of    organisms.     Comptes 
rend.  51,  298. 

1861.  "  Le9ons  de  chimie.     Eecherches  sur  la  dissymetrie 
moleculaire  des  produits  organiques." 

VAN'THOFF,  J.  H. 

September,  1874.  "Voorstel  tot  uitbreiding  der  tegen- 
woordig  in  de  schieikunde  gebruikte  strnctuur-formules 
in  de  ruiinte  ;  benevens  een  daarmee  sarnenhangende 
opmerking  omtrent  het  verband  tusschen  optisich  actief 
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bindingen." 

Utrecht.  8vo.  pp.  14. 

March,  1875.  A  republication  of  the  theories  given  in  the 
Dutch  pamphlet  with  slight  unimportant  changes. 
Bull  Soc.  Chim.  (Paris)  [2]  23,  295.  May,  1875.  "  La 
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1887.  "  Dix  annees  dans  1'historie  d'une  theorie  (Deuxieme 
edition  'de  La  chimie  dans  Fespace.)" 

1891.  "Chemistry  in  space."     Translated  and  edited  by 
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1892.  "  Stereo-chemie.     Nach  van't   Hoff's    '  Dix  annees 

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dans  Tliistorie  (Tune  theorie  '  neu  bearbeitet  von  W.  Meyer- 
hoifer. 

1894.  "Die  Lagerung  der  Atome  im  Raume,"  Zweite 
umgearbeitete  und  Vermehrte  auflage.  Mit  einem  Vor- 
wort  von  Dr.  Johannes  Wislicenus. 

1898.  "The  Arrangement  of  Atoms  in  Space/'  Second 
revised  and  enlarged  edition.  With  a  preface  by  Jo- 
hannes Wislicenus,  and  an  Appendix,  Stereo-chemistry 
among  Inorganic  substances  by  Alfred  Werner.  Trans- 
lated and  edited  by  Arnold  Eiloart. 

LEBEL,  J.  A. 

November,  1874.  Relation  between  atomic  formulas  and 
rotatory  power.  Bull  Soc.  chim.  (Paris)  [2]  22,  337. 

Derivatives  of  amyl  alcohol.  Ibid.  25,  545  ;  31,  104  ;  34-, 
129. 

"  Doubling  "  by  means  of  organisms  Comptes  rend.  87, 
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WISLICENUS,  JOHANNES. 

1873.     The  lactic  acids.     Ann.  167,  302. 

1887.     "  Ueber   die   raumlicbe  Anordnung  dcr  Atome  in 

organisachen  Molekulen  und   ihre  Bestimmung   in  geo- 

metrischisomeren    ungesattigten  Verbindungen"   Abh. 

kgl.     Sachs,     ges.  der  Wissench.     14,  1. 

Atomic    arrangement.     Verein   Deutsch.     Naturforsch. 

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tiger  Verbindungen  ineinander  bei  hoherer  Temperatur 

Leipzig  pp.  32,  4°. 

VON  BAEYER,  ADOLPH. 

The  "  Strain  "  theory,  Ber.  18,  2277.     The  constitution  of 
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Ergebnisse    und   Ziele  der  stereo-cheraischen   Forschung. 

Ber.  23,  567. 
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WUNDERLICH,  A. 

1886  "  Configuration  organischen  Molekule." 

(b)  Stereo-chemistry  of  Nitrogen. 

HANTZCH,  A.  AND  WERNER,  A. 

Ueber  r'aumliche  Anordnnng  der  Atome  in   stickstoffhalti- 

gen  Molekiilen.     Ber.  23,  11;  1243,  2764. 
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Ber.  23,  2322,  2325,  2332,  2769,  2773,  2776.  24, 13,  31,  36, 

51,  495,  1192,  3479,  4018.     26,  705,  1692,  1904,  2164. 
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Ber.  23,  2333,  2336;  25,  27. 
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Ansichtem  iiber  die  organische  Chemie,  pp.  79-81. 
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135 


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